Heredity Chapter 5



Heredity Chapter 5

Introduction:

For many years we have been breeding dogs, cats, horses and other animals and plants to produce offspring with desired traits. (Example: horses that can run fast, flowers that are prettier than others, dogs that have better mannerisms, etc.) We also try to breed plants that produce offspring that will produce a greater harvest, or resistant to specific diseases.

Can you think of other plants and animals that are bred or crossed for specific traits? Write a few down.

Mendel and his pea plants

If you look at yourself and try to compare yourself to others, most likely there is no one person exactly like you. Even if you have a twin, you are not exactly alike. You may resemble each other or your parents, but there is no one else exactly like you. That is what makes us all special and unique.

We will find out in this chapter, why we are not all exactly alike and how we get the traits we have that cause us to look, act, and behave different from other humans. We will focus on plants and animal traits to study this.

Gregor Mendel also wanted to know why organisms differed and what caused the differences to show up in offspring.

Why Don’t You Look Like A (Rhinoceros or any other Organism?)

Well, what do you think the answer to this topic is? If you said because your parents are not a rhinoceros or some other organism, then you are correct! You are who and what you are due to your parents. Heredity is the passing of traits, also called genes, from parents to offspring.

The person given credit for discovering this was Gregor Mendel. He worked with pea plants and bred pea plants to determine how traits are passed from one generation to another (parents to offspring). Mendel noticed some pea plants were always tall, some always short, and some always produced purple flowers, while others always produced white. Mendel questioned why this happened.

Mendel lived in a monastery during the time he studied the pea plant characteristics. Pea plants are self-pollinators. In other words, one pea plant flower has both male and female parts. The male parts produce pollen and the female part produces the egg. In this situation, pollen from one flower can fertilize the eggs of the same flower or the eggs of another flower on the same plant. With the help of an insect, wind, or other organism, the pollen can also be carried to a totally different pea plant where fertilization can occur.

When genes or traits are passed from the parent to the offspring, we say the offspring has inherited the genes from the parents. Mendel noticed, from his breeding of pea plants, that sometimes a trait from one generation would not show up in the second, but if he crossed (bred or mated) pea plants of the second generation, the traits would show back up in the third generation. Mendel noticed the same occurrences in other plants and animals also. To simplify all of his observations and try to learn what was occurring, he decided to work exclusively with the garden pea plant.

To make his research easier, he decided to study one trait at a time. He worked with one group of pea plants regarding the height of the plant from one generation to the next and in a separate experiment with different pea plants he worked with the color of flower the plants produced.

Recall from Chapter 1 (Scientific Method): When doing experiments it is best to test one variable at a time. This was what Mendel was doing by looking at height in one group of peas and flower color in a different group.

True-Breeding Plants

When a true-breeding plant pollinates itself, it always produces offspring with the same trait as the parent plant. Example: a true-breeding plant for the tall characteristic always produces tall offspring, a true-breeding plant for purple flowers always produces offspring with purple flowers.

Traits Mendel Looked at with the Pea Plants

Mendel noticed some of the pea plants produced wrinkled peas and some produced round peas. Mendel crossed a plant that produced wrinkled peas with a plant that produced round peas (these are the parent generation). The offspring from this cross is called the first generation. Mendel noticed the first generation plants produced all round peas. So Mendel concluded that the round trait must hide the wrinkled trait. Mendel called the trait that appeared; the dominant trait and the one that did not show up he called the recessive trait. Mendel then allowed the first generation to self-pollinate and the second-generation plants produced round and wrinkled seed, but he noticed for every round seed there was only one wrinkled seed.

In other words, the recessive trait showed up again in the second generation.

Mendel tested seven traits he found the pea plants to have and the following shows his results.

Characteristic Dominant Recessive

Flower color purple white

Seed color yellow green

Seed shape round wrinkled

Pod color green yellow

Pod shape smooth bumpy

Flower position along stem tip of stem

Plant Height tall short

Out of all of these traits Mendel conducted research on he noticed the result of the second-generation always had the dominant trait about three times more often than the recessive trait. So for every time a dominant trait showed up, the recessive trait showed up one time. This produces a “ratio” of 3:1.

Mendel’s Brilliant Idea

Mendel realized that the only way this could happen, was if each plant had two sets of instructions (now known as genes) for each characteristic. Mendel reasoned that each parent must contribute one gene each and the offspring would end up with two. The offspring therefore would have two forms of instruction (genes) for the same trait. Each individual form of a gene is known as alleles.

Proving His Idea was up to the Punnett Square

Mendel did not invent the Punnett Square, it was invented by a man with the last name of Punnett. A Punnett Square is a visual tool to see all the possible combinations of alleles that offspring can receive from their parents. When using a Punnett Square the dominant alleles are given a capital letter and the recessive alleles are assigned a lower case letter. Example: For flower color, Mendel noted that purple was dominant over the white recessive gene, therefore in a Punnett square purple would be assigned a “P” and white would be assigned a “p”.

How is a Punnett Square Set Up?

First, you draw a square that is large enough to divide into equal quarters and the quarters should be large enough to write the letter of the alleles into.

Try to follow this example on your own paper. We want to breed or cross a true-breeding purple pea plant “PP” with a true-breeding white pea plant “pp”. So, we would write “PP x pp”. Now for the square:

Draw a square large enough, maybe 2 inches by 2 inches and the divide it into equal units of four 1 inch by 1 inch squares. It should resemble the square below

| | |

| | |

Now we place our parents alleles on the outside of the square. One parent’s alleles will go across the top and the other will go down the side. Remember we are crossing PP x pp. You now should have a box that looks like this:

P P

| | |

| | |

p

p

Next we bring one parents alleles down and place them into the square and bring the other parents alleles across and place then into the square like this:

P P

| Pp | Pp |

| Pp | Pp |

p

p

Now we have all the possible genotypes of the offspring. In this situation all of the offspring will have Pp which is one alleles for the dominant purple and one allele for the recessive white. All of the pea plants will produce purple flowers, but carry the recessive gene for the white flower. This situation is called heterozygous (one allele for a different form of the same characteristic). This is the same type of work Mendel completed.

Now let’s see what happens if we take these offspring (first generation) and cross them together (Pp x Pp).

P p

| PP | Pp |

| Pp | pp |

P

p

Now, we see the genotype possibilities are PP, Pp, Pp, and pp. For the genotypes, one is homozygous dominant “PP”, two are heterozygous “Pp”, and one has the possibility of being homozygous recessive “pp”. for the phenotypes, three have the possibility of being purple and one has a possibility of being white. So now we have Mendel’s 3:1 ratio. So in this cross, there is a 75% probability that the pea will produce purple flowers and 25% probability the offspring plant will be white flowered.

All of these probabilities are random, in other words it is entirely random as to which alleles the offspring gets and each time we breed organisms with this possibility, there is the same chance to get the same outcome. Each fertilization or cross we conduct is independently random each time. Just because we cross a pea plant three times and get offspring that has purple flowers, does not guarantee we will get a white flowered pea plant on the fourth cross we complete.

What is probability? Probability is the mathematical chance that an event will happen or occur. Probability is usually expressed as a fraction or percentage (3/4 or 75%) or a ratio can be used sometimes (3:1).

Gregor Mendel published his findings in 1865, but his ideas were not given much attention until after his death about 30 years later. We now often referr to him as the “father of modern genetics”. Genetics is the study of passing of traits from one generation to another.

PRACTICE TIME HOMEWORK (1-10): Show your work on a separate sheet of paper and be prepared to turn in the problems for a grade.

Cross the Following:

1.     A true- breeding purple flowered pea plant with a heterozygous pea plant. You should have visualized or determined the true-breeding plant as “PP” and the heterozygous as “Pp”, so you have PP x Pp.

2.     Cross a heterozygous purple pea plant with a white pea plant.

3.     Use “T” for the tall trait, and “t” for the short trait. Cross a homozygous tall plant with a homozygous short plant.

4.     Cross a heterozygous tall pea plant with a homozygous tall pea plant.

5.     “R” is dominant for seed shape in peas, it means round. Wrinkled is the recessive trait. Cross a homozygous round pea plant with a homozygous wrinkled pea plant.

6.     Cross a heterozygous round pea plant with a homozygous round pea plant.

7.     Cross a heterozygous round pea plant with a homozygous wrinkled pea plant.

8.     Cross a homozygous smooth pod (smooth is dominant, bumpy is recessive) pea plant with a heterozygous smooth pod pea plant.

9.     Cross a homozygous smooth pod pea plant with a homozygous bumpy pea plant, then cross the first generation and show your second generation results. Tell me what the original parents (parent generation) genotype is in this scenario.

10. Now, brown hair is dominant over blonde hair in humans. What would the genotype of a homozygous brown haired person be? What about a heterozygous brown? What about a homozygous recessive blonde? Now cross a homozygous brown haired individual with a blonde haired individual. Cross two heterozygous brown haired individuals. Make sure you show all of your work clearly.

With this information you should be able to cross organisms with different forms of the same gene at this time. This is referred to a mono-cross. Mono means one. We will learn how to cross organisms with different forms of the same gene for two different genes, which are called a di-hybrid cross. Di meaning two.

 

 

Chapter 5 Section 2 Notes

Meiosis

Recall: there are two kinds of reproduction, 1) asexual and 2) sexual reproduction.

In asexual reproduction, only one parent is needed for reproduction to occur. This is how bacteria or prokaryotic cells reproduce. They copy their genetic information and then divide (binary fission).

In sexual reproduction, you must have two parent cells known as sex cells. In males, the sex cell is the sperm. In females, the sex cell is the egg. Remember that humans have 23 pairs of chromosomes. When we look at sex cells, each one will have only 23 chromosomes so when an egg and sperm unite, we end up with 23 pairs of chromosomes again (one from the mother and one from the father). In other words, human sex cells have half the usual number of chromosomes.

For sex cells to have 23 chromosomes, they must go through a process called meiosis. Meiosis produces new cells with half the usual number of chromosomes. Simply put, when sex cells are made, the chromosomes are copied and then the nucleus divides two times. The resulting cells (egg and sperm) have half the number of chromosomes found in a normal body cell.

Location, Location, Location

What does location have to do with genes? Well, a man by the name of Walter Sutton discovered that genes are located on chromosomes. Scientists have actually located specific genes and their locations on the chromosomes.

RECALL, RECALL, RECALL the steps of mitosis because meiosis is very similar. Review mitosis for a better understanding (Interphase, Prophase, Metaphase, Anaphase, and Telophase) know what happens in each phase that we have discussed in the past.

The Process of Meiosis (remember: meiosis is for sex cells).

The following is the phases of meiosis and the major event that happens in each phase.

Interphase: during this phase the chromosomes copy themselves.

Prophase 1: the nuclear membrane disappears and the chromosomes begin to pool into the center of the cell.

Metaphase 1: the chromosomes line up on the equator or middle of the cell.

Anaphase 1: the copied pairs of chromosomes pull away from each other toward the “poles” of the cell.

Telophase 1: the nuclear membrane reforms and the cell divide taking one complete pair of chromosomes into each new cell. (Now we have two cells with 23 pairs of chromosomes).

Prophase 2: the nuclear membrane disappears and the pairs of chromosomes in each cell pool in the cell.

Metaphase 2: the pair of chromosomes line up on the equator of each cell.

Anaphase 2: the pairs of chromosomes pull apart and move to the poles of each cell. (23 chromosomes move one direction and 23 move the other direction).

Telophase 2: the two cells form cleavage furrows, nuclear membranes reform and each cell divides to end up with four cells total and each cell has 23 chromosomes. Each new cell has half the number of chromosomes present in the original cell.

QUESTION: Does this process resemble anything we studied before? What does it resemble?

Meiosis review:

1. In a human, how many chromosomes are in the original single cell before meiosis?

2. In a human, how many times do chromosomes make copies of themselves in meiosis?

3. In a human, how many times do cells divide in meiosis?

4. In a human, how many chromosomes are in the cells at the end of meiosis?

5. In a human, how many chromosomes are in the cells at the end of mitosis?

Meiosis Review Answers

1.     In a human, there are 23 pairs of chromosomes or 46 chromosomes in the original single cell before Interphase begins. At the end of Interphase, there are 46 pairs of chromosomes or 92 chromosomes. At the end of Telophase 1 there are 23 pairs or 46 chromosomes in each cell. At the end of Telophase 2, there are 23 chromosomes in each cell and we have four cells total.

2.     In a human, chromosomes copy themselves only one time in meiosis.

3.     In a human, cells divide in meiosis two times. (Once in Telophase 1 and once in Telophase 2).

4.     In a human, there are 23 chromosomes in each of the four cells after the completion of meiosis.

5.     In a human, there are 23 pairs (46 chromosomes) in each cell at the end of mitosis.

Male or Female?

In humans we have 23 pairs of chromosomes. Twenty-two of those pairs are called autosomes, but one pair is called sex chromosomes. The reason they are called sex chromosomes is because these chromosomes determine if you are a male or female.

The sex chromosomes resemble an “X” or a “Y”. Remember we get one from our father and one from our mother, so we have two sex chromosomes. If a person has two X’s (XX), then the person is a female. If the person has an X and a Y (XY), then the person is a male.

In simple terms, the female can contribute an X sex chromosome and the male may contribute an X or a Y to the offspring. Experiment: Cross a male and female in a Punnett square to see the probability of having a male or female offspring. What did you get? Remember, it is random as to which sex chromosomes become fertilized together so we can only give probabilities that an offspring will be male or female by using Punnett squares.

We have become so technologically advanced that we can remove some of the cells from an embryo prior to birth and look at the sex chromosomes to determine if a male or female has been conceived. We also have ultrasound tests that can give us a picture of the fetus and knowing what to look for, we can tell if the child will be a male or female.

 

 

 

Chapter 5 Section 1 Worksheet (You must read to find the answers). (pages 106-110).

1.Why do you think you do not look exactly like anyone else?

2. When traits or genes are passed from parents to offspring, we call this h .

3. One scientist is known in the realm of science for working with pea plants in order to understand how traits are passed on from one generation to another.

This scientist is G M and was born in 1822 in Austria.

4. Mendel used the Scientific Method to conduct his work. Look at the example below:

Ask a Question: How are traits inherited?

Form a hypothesis: Inheritance has a pattern.

Test the Hypothesis: Cross true breeding plants and offspring.

Analyze the results: Identify patterns in inherited traits.

Draw Conclusions: Traits are inherited in predictable patterns.

As we study Mendel’s work, we will see that the example of the scientific method worked in this situation very well.

5. Draw and label the parts to the flower on page 107 in your book.

Explain what a true breeding plant is in your own words.

In Mendel’s first experiment, he called the trait that showed up (appeared) the d trait and the trait that “seemed to disappear” or recede into the background Mendel called the r trait.

What characteristics of the pea plant did Mendel look at? (page 109 tells the number and page 110 tells each characteristic).

In Mendel’s first experiment, he looked at pea seed shapes. What were the two shapes he wondered about?

What were the results of the first experiment?

What did Mendel do for his second experiment? What were the surprising results? How many round seeds turned up for every wrinkled seed?

Look at the numbers for the results of the second-generation crosses that Mendel completed. Do you see a pattern? What is the number of Dominant traits showing up for every Recessive trait? (Not an exact number with his data, but pretty close in all crosses).

 

Chapter 5 Study Sheet 2

Mendel realized that offspring must have t sets of instructions and that the offspring must get o set from each parent.

When each parent donates one set of instructions, we call these instructions ch . This means the fertilized egg will have two forms of the same gene for every characteristic (one from each parent). The two forms of a gene are known as a .

We now use Punnett Squares to understand Mendel’s concept (a picture is worth a thousand words). The actual genetic make-up of an organism is known as the organism’s g . The appearance of an organism is the organism’s p when we discuss heredity.

Recall (Dominant trait versus Recessive Trait).

We represent a dominant trait by using c letters, and a recessive trait by using l c letters.

Terms to recall (Heterozygous and Homozygous)

Use a Punnett Square to cross a homozygous purple flowered pea plant with a homozygous white flowered pea plant (PP x pp).

Recall from notes and class (distinguish between self pollination and cross pollination).

Recall the terms genotype and phenotype.

 

 

 

 

 

 

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