Genetics: Beyond Mendel

1. Genetics:Beyond Mendel IB Biology

2. Mendelian Genetics • This is the term used to describe the basic principles of inheritance for traits that are not inherited in complicated ways. • There are many exceptions to the principles we have learned with basic Mendelian genetics.

3. Incomplete Dominance • Sometimes, neither allele for a trait is dominant, so there is a blending of phenotypes • Example: • Homozygous red carnations are crossed with Homozygous white carnations and produce 100% pink carnation offspring RR x rr = Rr Red White Pink

4. Codominance • Sometimes, both alleles for a trait are dominant, resulting in offspring with both phenotypes. • Example: Crossing a homozygous white horse with a homozygous red horse produces a roan horse with a coat of red AND white hairs.

5. Multiple Alleles • This term describes traits for which there are more than 2 alleles • Example • There are three alleles for blood type: A (IA), B (IB) (codominant), and O (i) (recessive), allowing for 6 possible genotypes: • IA IA, IAi (type A phenotype) • IBIB, IBi (type B phenotype) • IA IB (type AB phenotype) • ii (type O phenotype)

6. Blood Types • Type A blood produces the A type glycoprotein on blood cell membranes (Type B produces B glycoprotein, type O produces a carbohydrate with no effect) • Giving someone with type A blood a type B transfusion will cause their immune system to recognize the type A glycoproteins as foreign antigens, attack them, and cause clotting and usually death for the patient

7. Blood Types • Those with type AB blood produce both glycoproteins (codominant), so they can receive A or B transfusions without an immune response • Those with type O blood are universal donors because the carbohydrate on their cell surfaces do not trigger an immune response, however, those with type O cannot receive any other form of blood

8. Epistasis • This occurs when one gene affects the phenotypic expression of a second gene. • Frequently occurs in the expression of pigmentation • One gene turns on (or off) the production of pigment, while a second gene controls either the amount of pigment produced, or the color of the pigment

9. Epistasis • Example: in mice, one gene codes for pigmentation, and another for the color of the pigment… • CC or Cc genotypes produce pigments, cc produces no pigments • BB or Bb makes the pigments black, bb makes the pigments brown (if present) • What color would a ccBb mouse be?

10. Pleiotropy • This occurs when a single gene has more than one phenotypic expression • Example: • The gene in pea plants that expresses the round or wrinkled texture of seeds also influences the phenotypic expressions of starch metabolism and water absorption • This is like killing 2 (or more) birds with one stone

11. Pleiotropy Example • Sickle-cell anemia is an example of a pleiotropic human blood disease. • It is caused by an allele that incorrectly codes for hemoglobin, causing normally round red blood cells to become sickle-shaped – leading to a painful death when homozygous recessive with that allele • Heterozygous individuals with that allele are more resistant to the mosquito-born pathogen malaria

12. Polygenic Inheritance • Many traits are not expressed in just 2 or 3 varieties, such as yellow and green pea seeds or A, B, O blood types • Your height, for example, is usually not just short or tall, but can be one of a nearly infinite continuum of possibilities within a certain range • Many genes are required to shape single complex phenotypes like height.

13. Linked Genes • The law of independent assortment only works for genes on different chromosomes • Linked genes are genes that reside on the same chromosome and cannot therefore segregate independently • Example: • Body color and wing structure genes in fruit flies are linked

14. Linked Genes • If the normal fruit fly body color is gray (B), while the mutant allele is expressed as black (b); and normal wings are full (V), while the mutant shriveled wings are vestigial (v)… • A dihybrid cross would typically reveal the following cross between this gray, normal-winged male (BbVv) and a black, vestigial-winged female (bbvv):

15. Normally: B’s and V’s sort independently Probabilities: ¼ BbVv, ¼ Bbvv, ¼ bbVv, and ¼ bbvv Linked: B’s and V’s do not sort so if B is on the same chromosome as V, they do not mix with b or v Probabilities: ½ BbVv, ½ bbvv

16. Experimental Probabilities: 41/100 BbVv, 41/100 bbvv, 9/100 Bbvv, 9/100 bbVv (41:41:9:9 ratio) . . . How? Crossing Over: Since genes cross over homologous chromosomes in prophase I of meiosis, in this case, about 18% of the time, we do see some of the unexpected combinations above (9% for each = 18% crossover rate). 82% of the time, normal linked combinations are revealed (41% for each expected result = 82%).

18. Linked Genes • The greater the distance between two genes on a chromosome, the more places between the genes that the chromosomes can break and thus the more likely the two genes will cross over during prophase I of meiosis. • So, we can think of every 1% of crossover rate as 1 map unit of distance separating the genes on a chromosome • This can help us to visualize the arrangement on a chromosome:

19. Linked Genes • Suppose you knew that for a fly with a genotype BBVVAA (where A is the apterous, or wingless mutant) the crossover frequency between B and V was 18%, between A and V was 12%, and between B and A was 6%. In what order do the genes lie on the chromosome, and how far apart are they?

20. Linked Genes • Hint: think of the 18% crossover rate between B and V as 18 map units apart (making these two the farthest apart from each other) • Draw out a possible solution:

21. Sex-Linkage • The one pair of homologous chromosomes in animals that does not have exactly the same genes are X and Y (sex chromosomes) • Traits whose genes are located on X (usually) or Y are determined, in part, by the sex of the offspring

22. Sex-Linkage • Example: red-green colorblindness (bb) is due to a gene on the X chromosome, normal sight will be represented by (BB or Bb): Normal Female Normal Male Carrier Female (normal) Colorblind Male Because their Y doesn’t have the colorblindness gene at all, males only need one copy to have the recessive phenotype (disorder); so they are much more likely to inherit sex-linked traits.

23. In the color slideshow, can you see a number hidden in the circle of dots?

24. X-Inactivation • During fetal development in females, one of the two X chromosomes will be randomly inactivated • It is formed into a Barr body and its genes are not expressed • Daughter cells from a cell which has inactivated one X will also inactivate the same X.

25. X-Inactivation • When X-inactivation begins, some cells will inactivate one X, and others will inactive the other X • It is unlikely that the same X will be inactivated in all initial embryo cells, but if it happens, then females can be subject to sex-linked disorders like hemophilia with only one copy of the mutated gene (like men)

26. Nondisjunction • If chromosomes do not separate correctly in meiosis, a parent can donate too many or too few chromosomes to their offspring:

27. Nondisjunction • Examples: • Down Syndrome: caused by an extra 21st chromosome (trisomy 21). Causes mental retardation, heart defects, respiratory problems, deformities, etc… • Turner Syndrome: caused by a missing X chromosome (X0). Results in a female with some physical abnormalities and sterility