Extensions of Mendelian Genetics

1. Extensions of Mendelian Genetics Incomplete Dominance is when a heterozygote expresses a phenotype intermediate between both alleles. For example, RR produces red flowers, Rr produces pink flowers and rr produces white flowers Codominance is when two alleles are expressed at the same time. ABO blood type is an example of this with both the dominant A and B being expressed in a type AB person. Multiple allelism occurs when there are more than two alleles of a gene. ABO blood types exhibit this.

2. Extensions of Mendelian Genetics ABO blood group has three alleles of one gene: IAand IB are codominant to each other; i is recessive to both other alleles.

3. Another blood group is called the Rh factor. Rh+ is caused by a dominant allele. Rh+Rh+ or Rh+Rh- Rh- is caused by 2 recessive alleles. Rh-Rh- Blood typing can be used to exclude potential parents. E.g., an AB parent can never have an O child and two Rh- parents can never have a Rh+ child. See Table 8.2 for compatibilities of blood types.

4. Pleiotropy is the ability of a single gene to cause multiple effects on the individual’s phenotype. Hemophilia is an example of pleiotropy. The inability to clot blood normally due to the absence of a clotting factor Gene for this clotting factor is on the X chromosome

5. Sex Determination and Sex Linkage Humans have 22 pairs of autosomes and one pair of sex chromosomes Women: two X chromosomes Men: one X and one Y chromosome

6. Sex Determination and Sex Linkage Sex-linked genes: genes located on the sex chromosomes X-linked: located on the X chromosome Y-linked: located on the Y chromosome SRY gene on Y chromosome leads to the development of the testes Males always inherit their X from their mother Males are more likely to express recessive X-linked traits than females due to carrying only 1 X. Females are less likely to express X-linked traits since they have to have 2 copies of the bad X’s.

7. Sex Determination and Sex Linkage • Carriers express the normal trait but are heterozygous, so they carry the allele for the recessive trait. • Hemophilia, red-green color blindness, and Duchenne Muscular dystrophy are examples of X-linked traits.

8. Pedigrees Pedigree: a family tree, showing the inheritance of traits through several generations Symbols commonly used in pedigrees are circles and squares Pedigrees reveal modes of inheritance

9. Pedigree for an autosomal dominant trait: Every affected person has an affected parent About half of the offspring of just one affected parent are also affected The phenotype occurs equally in both sexes

10. Pedigree for an autosomal recessive trait: Carriers: (Gg) have one allele for trait/disorder, they do not display the recessive phenotype Carriers: 2 parents that show no symptoms can have affected child: Cystic Fibrosis, Sickle Cell Anemia

11. Pedigree for an X-linked trait:

12. Practice Pedigrees: Steps • Identify if the trait is dominant, recessive (autosomal or x-linked), or other. • Write out all possible genotype as affected or unaffected. (AA, Aa, aa) • Fill in the easy sections first (obviously affected) Figure 7.13

13. What type on Inheritance is this? What are the genotypes?

14. What type on Inheritance is this? What are the genotypes?

15. DNA Fingerprinting No two individuals are genetically identical except for identical twins. Small differences in nucleotide sequences of their DNA This is the basis for DNA fingerprinting Unambiguous identification of people When sample size is small it is necessary to copy the genetic material to increase the quantity available for testing.

16. 7.6 DNA Fingerprinting • Steps in DNA fingerprinting: • DNA isolated from tissue sample • DNA cut into fragments with enzymes • DNAs of different sequences produce fragments of different sizes • Fragments separated on basis of size and visualized • Each person’s set of fragments is unique

17. DNA Fingerprinting Small amounts of DNA can be amplified using PCR (polymerase chain reaction) DNA is mixed with nucleotides, specific primers, Taq polymerase, and then is heated Heating splits the DNA molecules into two complementary strands Taq polymerase builds a new complementary strand DNA is heated again, splitting the DNA and starting a new cycle.

18. Each cycle, the amount of DNA doubles.

19. DNA is cut into fragments using restriction enzymes, which cut around DNA sequences called VNTRs (variable number tandem repeats).

20. Gel electrophoresis separates DNA fragments on basis of their sizes Electric current is applied to an agarose gel Smaller fragments run faster through the gel Fragments are transferred to a sheet of filter paper Labeled probe reveals locations of fragments containing VNTRs

21. Each person’s set of fragments is unique. All of a child’s bands must be present in one or both of the parents.

22. Who is more likely to suffer from colorblindness, a disorder caused by a recessive allele on the X-chromosome? • Males • Females • Babies • The elderly

23. What type of trait does this pedigree show? • Recessive trait • Sex-linked trait • Dominant trait • Codominant trait

24. Gel electrophoresis separates DNA fragments based on _______. • Shape • pH • Number of strands • Size

25. ______ is used to amplify DNA. • PCR • Gel electrophoresis • RFLP • VNTR