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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