Human Genetics Inheritance Patterns for Autosomal and Sex-Linked Genes Variations in Genetic Patterns

Loci, Genes and Alleles Locus = area on chromosome where gene is located (plural = loci)Locus = area on chromosome where gene is located (plural = loci) Gene = sequence of DNA that codes for a protein, gives rise to physical traitGene = sequence of DNA that codes for a protein, gives rise to physical trait Alleles = alternative forms of a geneAlleles = alternative forms of a gene –Dominant allele masks other alleles –Recessive allele is masked Locus 1 Locus 2 Locus 3

Separation of Alleles by Meiosis T t t tTTTT t t T Ttt Chromosomes duplicate Meiosis I Meiosis II dominant allele recessive allele centromere Each sex cell will carry only one allele for each gene

Autosomal Inheritance for One Gene Donald and Darla are members of a population at high risk for carrying the recessive allele, t, for Tay-Sachs disease. Individuals with at least one copy of the dominant allele, T, do not have the disease. A genetic counselor orders tests to determine the alleles that Donald and Darla have for this gene. The results show that they each have one copy of allele T and one copy of allele t. What are the chances that Donald and Darla could have a child with Tay-Sachs disease, inherited with the alleles tt?

Autosomal Inheritance of One Gene TT or Tt = no disease tt = Tay-Sachs disease Donald, no disease Darla, no disease TtTt T t T t TT Tt Tt tt Chances with each pregnancy: 75% no disease 25% Tay-Sachs disease phenotype = physical characteristic homozygous recessive homozygous dominant heterozygous (carrier) genotype = set of alleles

Autosomal Inheritance for Two Genes FF or Ff = Freckles ff = no freckles WW or Ww = widow’s peak ww = no widow’s peak George F f W w x F f W w Gina George F f W w x F f W w Gina Suppose George and Gina are both heterozygous for these traits, what are the chances that they will have a child with freckles but no widow’s peak? Fw fW FW fw Fw fW FW fw Each sex cell will carry one allele for each of the two genes, in all possible combinations.

Child with freckles but no widow’s peak Child with freckles and a widow’s peak Child with no freckles, no widow’s peak Child with no freckles but with a widow’s peak

Variations in Genetic Patterns: Incomplete Dominance Neither allele masks the other and both are observed as a blending in the heterozygote Example: Hair Curling HH = curly hair HH’= wavy hair H’H’ = straight hair

Incomplete Dominance Tina and Thomas both have wavy hair. What are the chances that they will have a child with straight hair? Tina and Thomas both have wavy hair. What are the chances that they will have a child with straight hair? Tina, wavy hair Thomas, wavy hair Chances with each pregnancy:

Variations in Genetic Patterns: Codominance Neither allele masks the other so that effects of both alleles are observed in heterozygotes without blending Example: ABO Blood Type Blood Type Genotypes Type A AA or Ao Type B BB or Bo Type AB AB Type O oo Alleles A and B are codominant. Alleles A and B are completely dominant over o. Effects of both alleles observed in phenotype Type A Type B Type AB Type O

Variations in Genetic Patterns: Multiple Alleles Three or more alleles exist for one trait [Note: A person can only carry any two of these alleles at once.] Examples: ABO Blood Type and also Rh Factor PhenotypeGenotype* Protein on Red Blood Cells Rh Positive RR or Rr Rhesus Protein Rh Negative rrNone *Although there are multiple R alleles, R 1, R 2, R 3, etc. all are completely dominant over all of the r alleles, r 1, r 2, r 3, etc. ABO Blood Type and Rh Factor are controlled by separate genes. They are inherited independently.

Example of Multiple Alleles and Codominance Type A, Rh positive x Type B, Rh negative (mother is Type O, Rh-) (mother is Type O, Rh-) (father is Type O, Rh-) Child with Type AB, Rh negative blood

Variations in Genetic Patterns: Polygenic Inheritance Many genes affect one trait Example: Skin color Number of Dominant Alleles Skin Color* (Phenotype)Genotypes % Pigmentation* 0Whiteaabb0-11% 1 Light Black Aabb or aaBb 12-25% 2 Medium Black AAbb or AaBb or aaBB 26-40% 3 Dark Black AABb or AaBB 41-55% 4 Darkest Black AABB56-78% *Based on a study conducted in Jamaica.

Example of Polygenic Inheritance Medium Black Woman X Darkest Black Man (her mother is white) (her mother is white)

Variations in Genetic Patterns: Sex Influenced Genes Autosomal genes that have different dominance patterns in males and females Example: Pattern Baldness Caused by a dominant allele in males, but a recessive allele in females bb BB or Bb bb

Sex-Influenced Genes Neither Frank nor Felicia have pattern baldness. Felicia’s father also did not have pattern baldness. If Frank and Felicia’s first child is a son, what are the chances that he inherited pattern baldness? Neither Frank nor Felicia have pattern baldness. Felicia’s father also did not have pattern baldness. If Frank and Felicia’s first child is a son, what are the chances that he inherited pattern baldness? Frank, no baldness Felicia, no baldness For a boy: For a girl:

Inheritance for Sex-linked Genes Hemophilic Male Non-hemophilic Female Female Chance for each pregnancy: Males carry only one copy of genes on the X chromosome Females can be homozygous or heterozygous these genes H = no hemophilia h = hemophilia (father is hemophilic)