Complex Patterns of Heredity

1. Complex Patterns of Heredity Chapter 12

2. Human Inheritance • Humans have 23 pairs of chromosomes • These are made of about 100,000 genes • Scientists usually study disease causing genes because they can easily be traced • They often prepare a Pedigree – a family record that shows how a trait is inherited over several generations.

3. Pedigree • Carriers – usually, Heterozygous; they do not express the recessive allele, but they pass it along to their offspring. Square = Male Circle = Female No shading = normal Shaded = displays trait Half/Half = Carrier

4. Pedigree A Pedigree of Hemophilia in the Royal Families of Europe

5. Simple Dominant Heredity • Many traits are inherited just as the rule of dominance predicts. • Remember that in Mendelian inheritance, a single dominant allele inherited from one parent is all that is needed for a person to show the dominant trait.

6. Patterns of Inheritance • Phenotypes repeated in a predictable pattern from generation to generation • Genetic Disorders – diseases or debilitating conditions that have a genetic basis

7. Patterns of Inheritance • Traits controlled by a Single Allele • More than 200 human traits are determined by a single dominant allele • Ex. Huntington’s Disease • More than 250 other traits are determined by homozygous recessive alleles • Both parents must have this allele in order for their offspring to have the disease. • Ex. Cystic Fibrosis & Sickle cell anemia

8. Sometimes Heredity Follows Different Rules • Incomplete Dominance: Appearance of a third phenotype • Codominance: Expression of both alleles • Multiple phenotypes from multiple alleles • Sex determination • Sex-linked inheritance • Polygenic Inheritance

9. Incomplete Dominance • With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits.  • RED Flower x WHITE Flower ---> PINK Flower

10. R = allele for red flowers W = allele for white flowers red x white ---> pink RR x WW ---> 100% RW

11. Codominance • The genetic gist to codominance is pretty much the same as incomplete dominance.  A hybrid organism shows a third phenotype --- not the usual "dominant" one & not the "recessive" one ... but a third, different phenotype.  • In COdominance, the "recessive" & "dominant" traits appear together in the phenotype of hybrid organisms.

12. Codominance • Example red x white ---> red & white spotted With codominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together.

13. R = allele for red flowers W = allele for white flowers red x white ---> red & white spotted RR x WW ---> 100% RW

14. Examples of Codominance • A very very very very very common phenotype used in questions about codominance is roan fur in cattle.  Cattle can be red (RR = all red hairs), white (WW = all white hairs), or roan (RW = red & white hairs together).  A good example of codominance. Another example of codominance is human blood type AB, in which two types of protein ("A" & "B") appear together on the surface of blood cells.

15. Codominance and Blood Types • Blood transfusion can only take place between two people who have compatible types of blood. • Human blood is separated into different classifications because of the varying proteins on the surface of blood cells. • These proteins are there to identify whether or not the blood in the individual's body is it's own and not something the immunity system should destroy.

16. Patterns of Inheritance • Traits controlled by Multiple Alleles • Controlled by 3 or more alleles of the same gene that code for a single trait • Ex. Blood types • A & B are codominant – both are expressed when together; and both are dominant to O. • A person can only have type O blood if they receive the “O” allele from both parents.

17. Human Blood Types

18. ABO Blood type and genetics • The protein's structure is controlled by three alleles; • i, IA and IB. • i, the recessive of the three, • IA and IB are both codominant when paired together. • If the recessive allele i is paired with IB or IA, it's expression is hidden and is not shown. • When the IB and IA are together in a pair, both proteins A and B are present and expressed. • The individual's blood type is determined by which combination of alleles he/she has. • There are four possible blood types in order from most common to most rare: O, A, B and AB. • O blood type represents an individual who is homozygous recessive (ii) and does not have an allele for A or B. • Blood types A and B are codominant alleles. • Codominant alleles are expressed even if only one is present. The recessive allele i for blood type O is only expressed when two recessive alleles are present. • Blood type O is not apparent if the individual has an allele for A or B. • Individuals who have blood type A have a genotype of IAIA or IAi and those with blood type B, IBIB or IBi, • An individual who is IAIB has blood type AB.

19. Blood type Chart

20. What is the chance that the couple from question 1 will have a child with type AB blood? *Show me your answers when you are finished. Keep these in your notes! Blood type practiceUse a Punnett Square! • A woman has type A blood. Her father has type O blood. The woman marries a man with type O blood. What is the chance that they will have a child with type A blood?

21. Multiple Phenotypes from Multiple Alleles • Although each trait that we have studied so far only has two alleles, it is common for more that two alleles to control a trait in a population • For instance, Pigeons – three colors possible (red, blue, chocolate) • However, each pigeon can have only two of these alleles • Complete P.S. Lab 12.2 to observe multiple alleles in how coat color in rabbits is inherited.

22. Sex determination • Remember that in humans the diploid number of chromosomes is 46, or 23 pairs. • There are 22 matching pairs of homologous chromosomes called autosomes. • The 23rd pair differs in males and females, they determine the sex of an individual (sex chromosomes) • X females (XX) • Y males (XY) Complete a punnett square to determine the expected ratio of males to females produced given their possible gamete contribution

23. Sex-linked inheritance • Traits controlled by genes located on sex chromosomes are called sex-linked traits • Read about Thomas Hunt Morgan’s research with fruit flies on pg 325. Complete a punnett square to show how the allele for red eye color is a sex-linked trait.

24. Patterns of Inheritance • Polygenic Traits • Most human characteristics are controlled by several genes (2 or more) • Ex. Skin color – 3 to 6 genes • Ex. Eye color • Some are also affected by the environment • Ex. Height – nutrition and disease

25. Eye Color Activity • http://www.athro.com/evo/gen/genefr2.html

26. Patterns of Inheritance • Sex-Linked Traits • Are found only on the X chromosome • Ex. Colorblindness (recessive) • Ex. Hemophilia (recessive)

27. Patterns of Inheritance • Sex-Influenced Traits • Influenced by male or female sex hormones • Ex. Patterned Baldness • Homozygous baldness-both will lose hair • Heterozygous-men will lose hair but women will not

28. Nondisjunction • Nondisjunction is the failure of homologous chromosome pairs to separate properly during meiosis. The result of this error is a cell with an abnormal (too few or too many) number of chromosomes.

29. Nondisjunction

30. Patterns of Inheritance • Disorders due to Nondisjunction • Monosomy (45 Chromosomes) • Trisomy (47 Chromosomes) • Trisomy-21 (Down’s Syndrome) • Klinefelter’s (XXY)-male w/ some female traits • Turner’s (XO)-female appearance • Single Y chromosome do not survive

31. Environmental Effects • Genes are inherited from parents, but sometimes their expression is modified by environmental factors. • An example is the snowshoe hare we discussed earlier in the year-these hares have dark fur in the summer and white fur in the winter.

32. Snowshoe Hare

33. Detecting Human Genetic Disorders • Genetic Screening – examination of genetic makeup • Karyotype: a picture of chromosomes grouped in pairs and arranged in sequence. • Screening of Blood: look for certain proteins • Genetic Counseling-medical guidance informing of problems that could affect their offspring.

34. Human Karyotype

35. Detecting HumanGenetic Disorders • Amniocentesis: removal of small amount of amnionic fluid surrounding the fetus • Chorionic Villi Sampling: tissue that grows between the mother’s uterus and the placenta (between the 8th and 10th week) • Screening Immediately after Birth: • Ex PKU (Phenylketonuria)-body cannot metabolize the amino acid phenylalanine • Special diet lacking phenylalanine

36. Karyotyping • http://www.biology.arizona.edu/human_bio/activities/karyotyping/karyotyping2.html