NonMendelian Genetics

1. NonMendelian Genetics Chapter 14: Mendel and the Gene Idea

2. Complex patterns of inheritance • The relationship between genotype and phenotype is rarely as simple as in Mendelian inheritance (controlled by dominant and recessive paired alleles) • Principles of segregation and independent assortment apply to more complex patterns of inheritance • Inheritance may deviate from simple Mendelian patterns in the following situations: • Alleles are not completely dominant or recessive • A gene has more than two alleles • A gene produces multiple phenotypes

3. Incomplete dominance • In complete dominance, heterozygous and homozygous dominant individuals have the same phenotype • With incomplete dominance, the phenotype of the heterozygous is intermediate between those of the two homozygotes • This intermediate occurs because neither allele of the pair is completely dominant

4. Incomplete Dominance • If you cross a white flower with a red flower that exhibit incomplete dominance the first generation (heterozygotes) will be all pink. • If you cross two of those heterozygotes you will get 1 red, 2 pink, 1 white flowers. (1:2:1 phenotypic ratio)

5. Incomplete dominance genetic problems • We can still use the Punnett Square to solve problems involving incomplete dominance. • The trick is to recognize when you are dealing with a question involving incomplete dominance. • There are two steps to this: 
 • 1) Notice that the offspring is showing a 3rd phenotype. The parents each have one, and the offspring are different from the parents. 
 • 2) Notice that the trait in the offspring is a blend (mixing) of the parental traits.

6. Incomplete Dominance Questions • 1. A cross between a black bird & a white bird produces offspring that are grey. The color of birds is determined by just two alleles. • a) What are the genotypes of the parent birds in the original cross? 
 • BB = black, BW= grey, WW = white • BB x WW • b) What is/are the genotype(s) of the grey offspring? • BW • c) What would be the phenotypic ratios of offspring produced by two grey birds? • 1 black, 2 grey and 1 white

7. Incomplete Dominance Questions • 2. The color of fruit for plant "X" is determined by two alleles. When two plants with orange fruits are crossed the following phenotypic ratios are present in the offspring: 25% red fruit, 50% orange fruit, 25% yellow fruit. • What are the genotypes of the parent orange-fruited plants? RY

8. Codominance • In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways • Codominant alleles cause the phenotypes of both homozygotes to be produced in heterozygote individuals. • In codominance both alleles are expressed. • For example, red cows crossed with white will generate roan cows. Roan refers to cows that have red coats with white blotches.

9. 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. • With incomplete dominance we get a blending of the dominant & recessive traits so that the third phenotype is something in the middle (red x white = pink). • In codominance, the "recessive" & "dominant" traits appear togetherin the phenotype of hybrid organisms. • red x white ---> red & white spotted

10. R R RW RW W W RW RW Codominance Punnett Squares • Some texts use letters & superscripts when dealing with codominance. • Others use different letters, noting the type of nonMendelian cross. • Let’s use the second method for our example • R = allele for red flowers 
 • W = allele for white flowers • red x white --> red & white spotted flowers • RR x WW ----> 100% RW • The symbols you choose to use don't matter, in the end you end up with hybrid organisms, and rather than one trait (allele) dominating the other, both traits appear together in the phenotype.

11. Codominance Questions • 1. Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull. • White = WW, roan = RW, red = RR • WW x RW = 1/2 white and 1/2 roan • 
2. A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together). 
 • a) What pattern of inheritance does this illustrate? Why? Codominance; both show • b) What percent of kittens would have tan fur if a tabby cat is crossed with a black cat? • TT=tan, TB = tabby, BB= black • TB x BB = 1/2 tabby and 1/2 black • 0%

12. Multiple Alleles • It is common for more than two alleles to control a trait in a population • Traits controlled by more than two alleles are said to have multiple alleles • A diploid individual can possess only two alleles of each gene

13. Multiple Alleles The number of alleles for any particular trait is not limited to four, there are instances in which more than 100 alleles are known to exist for a single trait

14. Multiple Alleles & Blood Types • Multiple Alleles govern blood type • Human blood types are determined by the presence or absence of certain molecules on the surfaces of red blood cells called antigens • As the determinant of blood type the gene I has three alleles: IA, IB, and i • Written A, B, and O • IA (or A) allele produces antigen A • IB (or B) allele produces antigen B • i (or O) produces no antigens • A & B codominant, but both dominant to O

15. Importance of Blood Typing • Incompatible blood types could clump together, causing death. • Disputed parentage • Example: If a child has type AB blood and its mother has type A, a man with type O blood could not be the father. • Why?

16. Blood Typing Practice • A woman with Type O blood and a man who is Type AB have are expecting a child. What are the possible blood types of the kid? • A, B • What are the possible blood types of a child who's parents are both heterozygous for "B" blood type? • B,O • What are the chances of a woman with Type AB and a man with Type A having a child with Type O? • None • A test was done to determine the biological father of a child.The child's blood Type is A and the mother's is B.Man #1 has a blood type of O, & Man #2 has blood type AB. Which man is the biological father? • Man #2

17. Pleiotropy • Most genes have multiple phenotypic effects, a property called pleiotropy • For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease • In the garden pea, gene for flower color also affects color of seed coat

18. Epistasis • In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus • For example, in mice and many other mammals, coat color depends on two genes • One gene determines the pigment color (B for black and b for brown) • The other gene (C for color and c for no color) determines whether the pigment will be deposited in the hair • Dominance = B masking b • Epistasis = cc masking BB or Bb or bb

19. Polygenic inheritancepoly = “many” ; genic = “genes” • More than one gene pair contributes to a phenotype • Effects of dominant alleles are additive • More dominant genes = increased effect • Number of dominant determines phenotype • Hair, eye and skin color (as well as height) are polygenic traits • Many disorders may be polygenic • Cleft palate, club foot, diabetes, schizophrenia, allergies, cancer

20. Skin color example • If skin color was related to 3 gene pairs • Dominant gene A, B or C produces pigment • Incompletely dominant to a, b or c • So # of dominant genes determines how much pigment is produced • AABBCC = lots of pigment • AaBbCc = middle range of pigment • aabbcc = very little pigment • 2 heterozygotes (AaBbCc) could have a child with any pigment range

21. Environmental Influences • Genes are also influenced by the environment • Temperature and Siamese cats • The darker colors on the extremities are due to a cooler body temperature • Gene that codes for production of the pigment in the Siamese cat only functions under cooler conditions • Many diseases, such as heart disease and cancer, have both genetic and environmental components

22. Pedigree • A pedigree is a family tree that describes the interrelationships of parents and children across generations • Inheritance patterns of particular traits can be traced • Can also be used to make predictions about future offspring • Many genetic disorders are inherited in a recessive manner • Recessively inherited disorders show up only in individuals homozygous for the allele • Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal

23. Pedigree Symbols

24. Albinism • Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair • If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low • Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele • Most societies and cultures have laws or taboos against marriages between close relatives

25. Cystic Fibrosis • Recessive condition • Cystic fibrosis is the most common lethal genetic disease in the US, striking one out of every 2,500 people of European descent • The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes • Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine

26. Sickle-cell disease • Recessive condition • Sickle-cell disease affects one out of 400 African-Americans • The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells • Symptoms include physical weakness, pain, organ damage, and even paralysis

27. Dominant Genetic Diseases • Some human disorders are caused by dominant alleles • Dominant alleles that cause a lethal disease are rare and arise by mutation • Achondroplasia is a form of dwarfism caused by a rare dominant allele • Huntington’s disease is a degenerative disease of the nervous system caused by a dominant allele • The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age

28. Genetic Tests • There are many genetic diseases that exist (way beyond the scope of what we will discuss) • Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease • Using family histories, they help couples determine the odds that their children will have genetic disorders • For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately

29. Amniocentesis • In amniocentesis, a long thin needle is used to remove amniotic fluid • The amniotic fluid contains fetal cells, which can be tested for genetic diseases • The DNA from fetal cells is karyotyped

30. Chorionic Villus Sampling • In chorionic villus sampling (CVS), a sample of the chorionic villus (developing placenta) is removed and tested • The chorionic villus cells contain the same genetic material as the fetus, making them fetal cells • The DNA from fetal cells is karyotyped

31. Karyotypes • Karyotypes (picture of chromosomes arrested during mitosis) are prepared, which determines: • # of chromosomes • Sex of individual • Extra or missing pieces of chromosomes

32. Other Genetic Tests • Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero • Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the US • Phenylketonuria (PKU) • Congential Hypothyroidism

33. Review Questions Name 3 examples of when inheritance patterns may not follow Mendelian rules. Explain, identify, and solve genetics problems involving incomplete dominance, codominance, & multiple alleles. Complete genetics problems involving blood types. Explain, differentiate between, and complete nontraditional genetics problems involving pleiotropy, epistasis, and polygenic inheritance. Explain the effect of the environment on the expression of our genes. Define and analyze a pedigree in order to answer inheritance questions. Identify the most common pedigree symbols. Identify the inheritance patterns and major characteristics of the following genetic conditions: albinism, cystic fibrosis, sickle-cell disease, achondroplasia, & Huntington’s disease. Explain the purpose, benefits, and risks of genetic testing. Differentiate between amniocentesis and chorionic villus sampling. Explain the purpose and use of a karyotype. List 3 pieces of information that can be obtained from a karyotype.