Beyond Mendel: Exceptions/Additions to Mendel’s Laws

1. Beyond Mendel: Exceptions/Additions to Mendel’s Laws AP Biology

2. Exceptions to Mendelian Genetics • Mendel chose traits in peas that showed 2 distinct forms. • Not all genes exhibit such simple inheritance. • Alleles interact • Genes interact • Segregation of genes on same chromosome • Mitochondrial DNA

3. Multiple Alleles • A population can have more than 2 alleles for a gene. • Ex. Labrador Retriever coat color • Determined by 1 gene with 4 alleles. • Even if more than 2 alleles exist in a population, any given individual can only have 2 of them • (1 from mother, 1 from father)

4. Multiple Alleles • Black is dominant to chocolate • B: black • b: chocolate • Yellow is recessive epistatic (when present, it blocks the expression of the black and chocolate alleles) • Yellow: E or e • * must be ee to produce a yellow lab

5. BBEE BbEE BBEe BbEe bbEEbbEe BBeeBbeebbee Possible Genotypes

6. Problem #1 • How do the multiple alleles act together to determine the coat color of a mouse? • There are 3 varieties of coat color: black, brown, and white • They are controlled by 4 different alleles (similar to the Labradors) • BbCc x BbCc • B=black, bb = brown • cc is epistatic (white)

8. Problem #2 • Determine the number of chocolate labs produced from a black female and a yellow male • BbEe x bbee

9. Codominance • Both alleles can be expressed • For example, red cows crossed with white will generate roan cows. • Roan cows have red coats with white blotches.

10. Incomplete Dominance • In some cases, an intermediate phenotype is shown. • Neither allele is dominant. • Ex. Snapdragons – flower color • 3 phenotypes: red, white, pink • Heterozygous condition results in pink flowers (the intermediate trait).

11. Incomplete Dominance - Snapdragons • A white (rr) snapdragon crossed with a red (RR) snapdragon produces all pink (Rr) offspring. • Two pinks crossed together (Rr x Rr) produce 1/4 white, 2/4 pink, 1/4 red

13. Sex-Linked Genes • Genes that are located on the X chromosome. • Females receive 2 alleles; males receive one. • Ex. Color blindness, hemophilia • Women can be carriers when they carry one gene for the disorder and one normal gene. • Carriers can have sons with the disorder.

14. Normal Male and Female Carrier

15. Problem #3 • Determine the probability of a woman with hemophilia having children with hemophilia assuming she marries a normal man.

16. Pleiotropy • Some single alleles have more than one distinguishable phenotypic effect. • This is called pleiotropy. • Ex. Coloration pattern and crossed eyes of Siamese cats • Both caused by the same allele. • Unrelated characters caused by the same protein produced by the same allele.

17. Siamese Cat • Siamese cats have a gene that codes for darker pigments - this gene is more active at low temperatures. • Parts of the body that are colder will develop the darker pigmentation - ears, feet tail of the siamese cats

18. Pleiotropy • Another example is the gene that causes pigment color in rats. • White rats also have very sensitive eyes and often become blind.

19. Pleiotropy • Another example is Marfan Syndrome. • Marfan Syndrome is a disease of the connective tissue. • Symptoms: tall & thin, long extremities, deficiencies in eyes and skeletal system, enlarged heart

20. Polygenic Traits • Individual heritable characters are often controlled by groups of several genes. • These genes are called polygenes. • Each allele intensifies or diminishes the phenotype. • Variation is continuous or quantitative (adding up) • Also called quantitative inheritance

21. Polygenic Traits • Examples: • Human hair, eye, and skin color • Height • Weight • Intelligence

22. Lethal Genes • Some genes are lethal when both alleles are present. • Lethality (death) can occur before or after birth.

23. Lethal Genes • Ex. The “creeper” allele in chickens, which causes the legs to be short and stunted. • Creeper is a dominant gene. • Heterozygous chickens display the creeper phenotype. • If 2 creeper chickens are crossed, one would expect to have ¾ creeper and ¼ normal • Instead, the ratio is 2/3 creeper and 1/3 normal.

24. Lethal Genes – Creeper Chickens

25. Lethal Genes • Mexican hairless dogs result from a mutation in a gene that shows lethality. • hh hairy normal trait • Hh hairless one mutation present • HH lethal two mutations = lethal

26. Manx Cats • Cats possess a gene for producing a tail. • The tailless Manx phenotype in cats is produced by an allele that is lethal in its homozygous state. • The allele interferes with normal spinal development, in heterozygous cats this results in lack of a tail.

27. Blood Types – Multiple Alleles and Codominance • 4 blood types (humans) • A, B, AB, and O • Blood type is controlled by 3 alleles • A, B, O • O is recessive (must have two O alleles to have blood type O) • A and B are codominant (if inherit an A and B, blood type is AB) • Crosses involving blood type often use an I to represent the alleles

28. Blood Types • The blood type determines what antibodies are located within the blood. • Type A blood has type B antibodies • If Type B blood is put into their body, their immune system reacts and antibodies clump the blood – can cause death • Type AB blood has no antibodies, any blood can be donated to them; they are “universal acceptors” • Type O blood has no antigens, antibodies in the blood do not react to type O blood, they are “universal donors”

31. Blood Type Cross