Lecture 46 – Lecture 47 – Lecture 48 Complex Patterns of Inheritance Ozgur Unal 1

2  In the pictures below, you see two snapdragon flowers (one red, RR, and one white, rr).  When you cross them, the daughter flowers are all pink, Rr.  How can you explain this?  Which allele is dominant? Which allele is recessive?

3  In incomplete dominance, the heterozygous phenotype is an intermediate phenotype between the two homozygous phenotypes.  When the heterozygous F1 generation snapdragon plants are allowed to self-fertilize, the flowers are red, pink and white in a 1:2:1 ratio.

4  There is another type of complex inheritance pattern called codominance.  In codominance, both alleles are expressed in the heterozygous condition.  Example: Sickle cell disease.  A person with a sickle cell disease has abnormal red blood cells  C-shaped elongated red blood cells  If a person is homozygous for the sickle cell trait, all red blood cells have this abnormal shape.  If a person is heterozygous for the sickle cell trait, some red blood cells are normal and some red blood cells are C-shaped.  This means that the allele for normal blood cells and the allele for sickle cell are both dominant  codominant

5  Example: Checkered chicken  Consider the black chickens, BB and white chickens, WW.  When these two are allowed to mate, how do you think the daughter chickens look like?  Since black and white are codominant, the daughter chickens will have black and white feathers.

6  So far, you have learned about inheritance involving two forms of allele for a trait (yellow-green seed color etc).  Some traits are determined multiple (three or more) alleles.  Example: Blood type  ABO blood groups have three forms of alleles (AB markers):  I A is blood type A  I B is blood type B  i is blood type O  i is recessive to I A and I B.  However, I A and I B are codominant.  Blood also has Rh factor, inherited from each parent.  Rh factors are either positive or negative (two alleles)  Rh+ is dominant to Rh-.

7  Example: Coat color of rabbits  Coat color of rabbits is determined by 4 alleles: C, c ch, c h, c  Multiple alleles can demonstrate a hierarchy of dominance.  Allele C is dominant to c ch, which is dominant to c h, which is dominant to c  C > c ch > c h > c  C  full color  c ch  chincilla  c h  himalayan  c  albino  Check out Figure 11.7

8  What are the differences between the following Labrador retrievers?  The fur color of these dogs are determined by the interaction of two sets of alleles (E, e and B, b)  The allele E determines whether the fur has a dark pigment  The allele B determines how dark the pigment is.  e allele masks the effects of the dominant B allele  epistasis  The interaction in which one allele hides the effect of another allele is called epistasis.

9  Each cell in your body, except for gametes, contains 46 chromosomes, or 23 pairs of chromosomes.  One pair of chromosomes, called sex chromosomes, determines an individual’s gender.  There are two types of sex chromosomes: X and Y  The other 22 pairs of chromosomes are called autosomes.  The offspring’s gender’s determined by the combination of sex chromosomes during fertilization.

10  Consider the X and Y chromosomes  Y is shorter than X  X chromosome carries a variety of genes that are necessary for the development of both males and females.  Y chromosome mainly has genes that relate to the development of male characteristics.  Because females have two X chromosomes, it seems as though females get two doses of X chromosome and males get one dose.  To balance the difference, one of the X chromosomes stops working in each of the female’s body cells  dosage compensation or X-inactivation  Which X chromosome stops working in each body cell is a completely random event.

11  Example: Calico cat  The coat of the calico cat has black and orange colors.  These different colors are a result of random inactivation of a particular X chromosome.  Orange patches are formed when X chromosome carrying the black coat color is inactivated.  Black patches are formed when X chromosome carrying the orange coat color is inactivated.

12 Barr Bodies:  The inactivated X chromosomes can be observed in cells.  In 1949, Canadian scientist Murray Barr observed inactivated X chromosomes in female calico cats.  The darkly stained, inactivated X chromosomes are called Barr bodies.

13  Look at the picture below. Can you read the number in the picture?  Red-green color blindness is a sex-linked trait.  Traits controlled by genes located on the X chromosome are called sex-linked traits (also called X-linked traits)  Since males have only one X chromosome, they are affected by recessive X-linked traits more often than females.  Females likely would not express a recessive X-linked trait.

14 Red-green color blindness:  The trait for color blindness is a recessive X-linked trait.  8% of males in the US have red-green color blindness.  Consider a mother carrier for color blindness and a father who is not color blind.  Their daughter will not have color blindness.  Their son might be color blind (50% chance)

15 Hemophilia:  Hemophilia, another recessive sex-linked trait, is characterized by delayed clotting of the blood  results in death when the blood flow cannot be stopped.  More common in males than females.  Check out Figure for hemophilia disease in the Royal Family.

16  Name all the eye colors you know.  How do your eye color compare to your parents’ and siblings’ eye colors?  Many phenotypic traits, such as eye color, arise from interaction of multiple pairs of genes  polygenic traits  Example: Skin color, height, eye color, fingerprint are polygenic traits.

17  The environment also has an effect on phenotype.  Example: Tendency to develop a heart disease can be inherited, but diet and exercise can reduce the seriousness and the risk.  Sunlight, water and temperature are examples of some environmental factors. Sunlight and water:  Lack of sunlight affects flowering plants  no flowers  Many plants lose their leaves in response to water deficiency Temperature:  Arctic fox changes fur color with the seasons

18  Twin studies:  Identical twins help scientists study inheritance patterns  Identical twins also allow scientists separate genetic contributions from environmental contributions.  The percentage of twins who both express a given trait is called a concordance rate.  Check out Figure for concordance rates for different traits.