NonMendelian Genetics

1. NonMendelian Genetics Heredity Part 2

2. Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical • In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties • In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways Degrees of Dominance

3. Fig. 14-10-3 P Generation Red White CRCR CWCW CR CW Gametes Pink F1 Generation CRCW 1/2 1/2 CR CW Gametes Sperm 1/2 1/2 CR CW F2 Generation 1/2 CR CRCW CRCR Eggs 1/2 CW CRCW CWCW

4. Dominant alleles are not necessarily more common in populations than recessive alleles • The allele for trait of having extra fingers or toes is dominant to the allele for the more common trait of five digits per appendage • But only 1 baby out of 400 in the United States is born with extra fingers or toes • Recessive trait is more common Frequency of Dominant Alleles

5. Most genes exist in populations in more than two allelic forms • For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i. • The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither Multiple Alleles

6. Fig. 14-11 Carbohydrate Allele Phenotype (blood group) Red blood cell appearance Genotype IA A B IB i none (a) The three alleles for the ABO blood groups and their associated carbohydrates IAIA or IA i A B IBIB or IB i AB IAIB ii O (b) Blood group genotypes and phenotypes

7. Quantitative characters are those that vary in the population along a continuum • Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype • Skin color in humans is an example of polygenic inheritance Polygenic Inheritance

8. Fig. 14-13  AaBbCc AaBbCc Sperm 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 Eggs 1/8 1/8 1/8 1/8 Phenotypes: 1/64 6/64 15/64 20/64 15/64 1/64 6/64 Number of dark-skin alleles: 2 6 0 3 4 5 1

9. Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype • The norm of reaction is the phenotypic range of a genotype influenced by the environment • For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity Environmental Impact on Phenotype

10. Fig. 14-14

11. Fig. 14-15 Key Affected male Male Mating A pedigree is a family tree that describes the interrelationships of parents and children across generations Inheritance patterns of particular traits can be traced and described using pedigrees Offspring, in birth order (first-born on left) Affected female Female 1st generation (grandparents) Ww Ww ww ww 2nd generation (parents, aunts, and uncles) Ww Ww Ww ww ww ww 3rd generation (two sisters) WW ww or Ww Pedigrees can also be used to make predictions about future offspring We can use the multiplication and addition rules to predict the probability of specific phenotypes No widow’s peak Widow’s peak (a) Is a widow’s peak a dominant or recessive trait? 1st generation (grandparents) Ff Ff Ff ff 2nd generation (parents, aunts, and uncles) ff ff FF Ff Ff or Ff ff 3rd generation (two sisters) FF ff or Ff Attached earlobe Free earlobe (b) Is an attached earlobe a dominant or recessive trait?

12. 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 (i.e., pigmented) Recessive Disorders

13. 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 Dominant Disorders

14. Many diseases, such as heart disease and cancer, have both genetic and environmental components • Little is understood about the genetic contribution to most multifactorial diseases Multifactorial Diseases

15. Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease • Using family histories, genetic counselors 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 Genetic Testing & Counciling

16. In amniocentesis, the liquid that bathes the fetus is removed and tested • In chorionic villus sampling (CVS), a sample of the placenta is removed and tested • Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero Fetal Testing

17. Fig. 14-18 Amniotic fluid withdrawn Centrifugation Fetus Fetus Suction tube inserted through cervix Placenta Placenta Chorionic villi Cervix Uterus Fluid Bio- chemical tests Fetal cells Several hours Fetal cells Several hours Several weeks Several hours Several weeks Karyotyping (b) Chorionic villus sampling (CVS) (a) Amniocentesis

18. Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the United States Newborn Screening