Chapter 20

1. Chapter 20 Patterns of Genetic Inheritance

2. Points to Ponder • What is the genotype and the phenotype of an individual? • What are the genotypes for a homozygous recessive and dominant individuals and a heterozygote individual? • Be able to draw a punnett square for any cross (1-trait cross, 2-trait cross and a sex-linked cross). • What are Tay-Sachs disease, Huntington disease, sickle-cell disease, and PKU? • How are each of the above inherited? • What is polygenic inheritance? • What is a multifactorial trait? • What is sex-linked inheritance? • Name 3 X-linked recessive disorders. • What is codominance? • What is incomplete dominance? • What do you think about genetic profiling?

3. 20.1 Genotype and phenotype These traits are genetically inherited Answer these questions about your inheritance: • Do you have a widow’s peak? • Are your earlobes attached or unattached? • Do you have short or long fingers? • Do you have freckles? • Can you roll your tongue? • Do you have Hitchhiker’s thumb?

4. 20.1 Genotype and phenotype Genotype Genotype – specific genes for a particular trait written with symbols • Alleles: alternate forms of a specific gene at the same position (locus) on a gene (e.g. allele for unattached earlobes and attached lobes); alleles occur in pairs • Dominant gene: will be expressed and will mask a recessive gene (Tt or TT) • Recessive allele: allele that is only expressed when a gene has two of this type of allele • Homozygous dominant genotype: 2 dominant alleles (TT or AA) • Homozygous recessive genotype: 2 recessive alleles (tt or aa) • Heterozygous genotype: one dominant allele and one recessive allele (Tt or Aa)

5. 20.1 Genotype and phenotype Phenotype Phenotype – the physical or outward expression of the genotype GenotypePhenotype EE unattached earlobe Ee unattached earlobe ee attached earlobe What are your genotype and phenotype?

6. 20.1 Genotype and phenotype Understanding genotype & phenotype

7. 20.2 One-and Two-trait inheritance What about your inheritance?

8. 20.2 One-and Two-trait inheritance Crosses • One-trait cross – considers the inheritance of one characteristic e.g. WW x Ww • Two-trait cross – considers the inheritance of two characteristics e.g. WWTT x WwTT • Gametes only carry one allele, so if an individual has the genotype Ww what are the possible gametes that this individual can pass on? Answer: either a W or a w but not both Another example:

9. Punnett squares are the use of a grid that diagram crosses between individuals by using the possible parental gametes These allow one to figure the probability that an offspring will have a particular genotype and phenotype 20.2 One-and Two-trait inheritance Punnett squares

10. 20.2 One-and Two-trait inheritance Practicing punnett squares • What would a punnett square involving a man (M) with a genotype Ff and a women (F) with a genotype Ff look like? F – freckles f – no freckles eggs sperm

11. 20.2 One-and Two-trait inheritance Practicing ratios • Genotypic ratio: the number of offspring with the same genotype • Phenotypic ratio: the number of offspring with the same outward appearance • What is the genotypic ratio? 1: 2: 1 (1 FF: 2 Ff: 1 ff) • What is the phenotypic ratio? 3: 1 (3 with freckles and 1 with no freckles) eggs sperm

12. 20.2 One-and Two-trait inheritance Monohybrid crosses Monohybrid cross – an experimental cross in which parents are identically heterozygous at one gene pair (e.g. Aa x Aa)

13. 20.2 One-and Two-trait inheritance Possible gametes for two traits

14. Dihybridcross – an experimental cross usually involving parents that are homozygous for different alleles of two genes and results in a 9:3:3:1 genotypic ratio for the offspring 20.2 One-and Two-trait inheritance Dihybrid cross (a type of two-trait cross)

15. What would the punnett square look like for a dihybrid cross between a male that is WWSS and a female that is wwss? 20.2 One-and Two-trait inheritance Practicing a punnett square for a 2-trait cross

16. 20.2 One-and Two-trait inheritance Autosomal recessive disorder • Individuals must be homozygous recessive to have the disorder

17. 20.2 One-and Two-trait inheritance Autosomal dominant disorder • Individuals that are homozygous dominant and heterozygous will have the disorder

18. 20.2 One-and Two-trait inheritance Genetic disorders of interest • Tay-Sachs disease: lack of the enzyme that breaks down lipids in lysosomes resulting in excess and eventually death of a baby • Cystic fibrosis: Cl- do not pass normally through a cell membrane resulting in thick mucus in lungs and other places often causing infections • Phenylketonuria (PKU): lack of an enzyme needed to make a certain amino acid and affects nervous system development • Sickle-Cell disease: red-blood cells are sickle shaped rather than biconcave that clog blood vessels • Huntington disease: huntingtin protein has too many glutamine amino acids leading to the progressive degeneration of brain cells

19. 20.2 One-and Two-trait inheritance Genetic disorders

20. 20.3 Beyond simple inheritance Polygenic inheritance • Polygenic traits - two or more sets of alleles govern one trait • Each dominant allele codes for a product so these effects are additive • Results in a continuous variation of phenotypes • Environmental effects cause intervening phenotypes • e.g. skin color ranges from very dark to very light • e.g. height vary among • Multifactorial trait – a polygenic trait that is particularly influenced by the environment • e.g. skin color is influenced by sun exposure • e.g. height can be affected by nutrition

21. 20.3 Beyond simple inheritance Polygenic inheritance

22. Himalayan rabbit’s coat color influenced by temperature There is an allele responsible for melanin production that appears to be active only at lower temperatures The extremities have a lower temperature and thus the ears, nose paws and tail are dark in color 20.3 Beyond simple inheritance Demonstrating environmental influences on phenotype

23. 20.3 Beyond simple inheritance Incomplete dominance • Occurs when the heterozygote is intermediate between the 2 homozygotes • Example: (curly hair) CC x SS (straight hair) CS (wavy hair)

24. 20.3 Beyond simple inheritance Codominance • Occurs when the alleles are equally expressed in a heterozygote • Example: (Type A blood) AA x BB (Type B blood) AB (Type AB blood that has characteristics of both blood types)

25. 20.3 Beyond simple inheritance Multiple allele inheritance • The gene exists in several allelic forms • A person only has 2 of the possible alleles • A good example is the ABO blood system • A and B are codominant alleles • The O alleles is recessive to both A and B therefore to have this blood type you must have 2 recessive alleles

26. 20.3 Beyond simple inheritance Multiple allele inheritance Based on what you know what type of blood would each of the following individuals have in a cross between Ao and Bo? possible genotypes: phenotypes: AB Type AB blood Bo Type B blood Ao Type A blood oo Type O blood

27. 20.3 Beyond simple inheritance Blood type inheritance

28. 20.4 Sex-linked inheritance Sex-linked inheritance • Traits are controlled by genes on the sex chromosomes • X-linked inheritance: the allele is carried on the X chromosome • Y-linked inheritance: the allele is carried on the Y chromosome • Most sex-linked traits are X-linked

29. Cross: XBXb x XBY Possible offspring: XBXB normal vision female XBXb normal vision female XBY normal vision male XbY normal vision male 20.4 Sex-linked inheritance X-linked inheritance: Color blindness

30. 20.4 Sex-linked inheritance X-linked disorders • More often found in males than females because recessive alleles are always expressed • Most X-linked disorders are recessive: • Color blindness: most often characterized by red-green color blindness • Muscular dystrophy: characterized by wasting of muscles and death by age 20 • Hemophilia: characterized by the absence of particular clotting factors that causes blood to clot very slowly or not at all

31. 20.4 Sex-linked inheritance X-linked disorders

32. 20.4 Sex-linked inheritance X-linked disorders: Hemophilia

33. 20.4 Sex-linked inheritance Bioethical focus: Genetic profiling • Genetic profiling is a way to look for genetic disorders that you may have now or in the future • Discrimination concerns: • Could insurance companies use this to increase rates or not insure you? • Could an employer not hire you based on this knowledge? • Employer concerns: • Could the government mandate they provide an environment for each employee’s need in order to prevent illness or should employees be required to move from an area or job that could cause future disease? • Public benefits: • Most believe this could lead to better preventative care • Having this type of information can allow complex studies that can further our understanding of disease