Patterns of Gene Inheritance - Biology



Patterns of Gene Inheritance

(Chapters 10 & 11)

Gene: unit of heredity

• Individual genes, or DNA segments, contain the genetic blue-print which is ultimately expressed in our hair color, blood type, etc.

• DNA is packaged in chromosomes

Homologous chromosomes: chromosomes of the same size and shape that contain genes for the same trait

• Each somatic cell contains 23 pairs of homologous chromosomes (diploid number)

o 1 pair of the chromosomes are sex chromosomes (XY or XX) that determine gender

o other 22 pairs of chromosomes called autosomal chromosomes guide the expression of every other trait

Mendel’s Law of Segregation

• each individual has to factors for each trait

• factors separate during meiosis

• each gamete contains one factor for each trait

Terminology of genetics

Alleles: genes coding for the same traits on each pair of homologous chromosomes

• Alleles may be identical or different in their influence

• For example, member of a gene pair, or alleles, coding for hairline shape on your forehead may specify straight across or widow’s peak

• When both alleles in a homologous chromosome pair have the same expression, the individual is said to be homozygous for that trait

o Homozygous: when two alleles controlling a single trait are identical

• When alleles differ in their expression, the individual is heterozygous for that trait and typically one of the alleles, called the dominant allele will exert its effects

o Heterozygous: when two alleles controlling a single trait differ in their expression for the trait

Dominant allele: gene that is always expressed if present

Recessive allele: gene that is not expressed in the presence of a dominant allele

Genotype: refers to an individual’s genetic makeup (i.e., whether homozygous or heterozygous for the various alleles)

• two letters are used to designate each trait since homologous chromosomes each contain genes for a particular trait

Trait: hairline shape

W = Widow’s peak (dominant allele)

w = Straight hairline (recessive allele)

Genotype written as: Phenotype

Homozygous dominant WW Widow’s peak

Heterozygous Ww Widow’s peak

Homozygous recessive ww straight hairline

Phenotype: characteristics of the individual based on expression of the genes

Punnett Square:

Method used to calculate probable results of a genetic cross ( i.e., predict genotypes and phenotypes of offspring

Example: Trait ( earlobe shape

D = detached earlobes (dominant allele)

d = attached earlobes (recessive allele)

Mother = homozygous dominant

Father = homozygous recessive

What type of earlobe shape will their children have?

• Heterozygous ( detached earlobes

If these children (Dd) reproduce with individuals of the same genotype, what type of earlobe shape will their children have?

Other Forms of Inheritance:

• Certain traits follow the rules of simple Mendelian inheritance (i.e., dominant-recessive type of inheritance), but other inheritance patterns exist for many traits

Multiple Alleles: more than two alleles control a particular trait

For example: blood type

• Blood types: A, B, AB, and O are phenotypes caused by three different alleles

• Each person has only two of the three alleles

• A and B are dominant and will be fully expressed in the presence of the other ( called codominance

• Type O is recessive

Phenotype Genotype

A AA, Ao

B BB, Bo

AB AB

O oo

Blood typing can sometimes aid paternity suits, but they can only suggest that an individual is the supposed father. In most cases they can only exclude possible paternity.

♀ Ao x ♂ Bo

♀ Bo x ♂AB

Incomplete dominance

• the heterozygote has a phenotype intermediate between the homozygous dominant and the homozygous recessive

• Example: Sickle-Cell Anemia

Genetic Disorders

• Sickle-cell gene demonstrates incomplete dominance

• Autosomal dominant disorders

o Genotype AA or Aa (heterozygous) will have disorder

o Example: Huntington’s Disease

▪ Neurological disorder resulting in degeneration of brain cells

• Autosomal recessive disorders

o Genotype aa will have disorder, but Aa is a carrier

o Example: cystic fibrosis

▪ Most common lethal genetic disorder in US

▪ 1 in 20 caucasians are carriers

o Carrier: individual who has the abnormal recessive gene ( not expressed since they are heterozygous

• Nondisjunction:

o failure of homologous chromosomes or sister chromatids to separate during meiosis in the process of oogenesis or spermatogenesis

Result: abnormal number of chromosomes inherited by gametes

• offspring inherit an extra chromosome or are missing a chromosome

• procuces syndrome: a group of symptoms that appear together and indicate the presence of a particular disorder

• Autosomal Nondisjunction:

• Down Syndrome ( usually have three copies of chromosome 21 because the egg had two copies instead of one

• Sex Chromosome Nondisjunction

• Turner syndrome (XO)

• Triplo-X (XXX)

Sex-linked Inheritance (or X-linked inheritance)

XX ( female

XY ( male

The gender of a newborn is determined by the father:

o If a Y containing sperm fertilizes the egg, then the XY combination results in a male

o If an X-containing sperm fertilizes the egg, the XX combination results in a female

• X-linked inheritance ( Body traits that are inherited from the X-chromosome

• A male always receives sex-linked condition from his mother

• The Y chromosome from the father does not carry an allele for the trait

• Usually the trait is recessive ( the female must receive two alleles before she has the condition

• Common X-linked recessive disorders:

o Color blindness

o Hemophilia: blood clotting disorder

o Muscular dystrophy: degeneration of muscle tissue

Example: red-green color-blindness

XB = normal color vision

Xb = color blind

♀ XBXb (carrier) ♂ XBY (normal)

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