Observable Patterns of Inheritance Chapter 20

Earlobe Variations If you have attached earlobes, you inherited two copies of the recessive allele If you have detached earlobes, you may have either one or two copies of the dominant allele

Gregor Mendel Strong background in plant breeding and mathematics Using pea plants, found indirect but observable evidence of how parents transmit genes to offspring

Alleles Different molecular forms of a gene Arise by mutation Dominant allele masks a recessive allele that is paired with it

Allele Combinations Homozygous –having two identical alleles at a locus –AA or aa Heterozygous –having two different alleles at a locus –Aa

Genetic Terms A pair of homologous chromosomes A gene locus A pair of alleles Three pairs of genes

Genotype & Phenotype Genotype refers to particular genes an individual carries Phenotype refers to an individual’s observable traits Cannot always determine genotype by observing phenotype

Tracking Generations Parental generation P mates to produce First-generation offspring F 1 mate to produce Second-generation offspring F 2

Mendel’s Theory of Segregation An individual inherits a unit of information (allele) about a trait from each parent During gamete formation, the alleles segregate from each other

Segregation C CCcc (meiosis) Parents: Gametes: (meiosis) Ccc AA parent produces only A gametes; aa parent produces only a gametes

Probability The chance that each outcome of a given event will occur is proportional to the number of ways that event can be reached

Punnett Square of a Monohybrid Cross Female gametes Male gametes C c C c CCCc cc Dominant phenotype can arise three ways, recessive only one

Test Cross Individual that shows dominant phenotype is crossed with individual with recessive phenotype Examining offspring enables you to determine the genotype of the dominant individual

Punnett Squares of Test Crosses c C ccc Cc cc c C CCc Two phenotypesAll dominant phenotype

Independent Assortment Mendel concluded that the two “units” for the first trait were to be assorted into gametes independently of the two “units” for the other trait Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis

Independent Assortment Metaphase I: Metaphase II: Gametes: 1/4 AB1/4 ab1/4 Ab1/4 aB AAAA AAAA AAAA BB BB BB BB BBBB aaaa aaaa aaaa bbbb bbbb bbbb OR

Dihybrid Cross Experimental cross between individuals that are homozygous for different versions of two traits

Dihybrid Cross - F 1 Results ccdd (smooth chin, no dimples) CCDD (chin fissure, dimples) CD cd CcDd Parents: Gametes: F 1 offspring:

Allele Combinations in F 2 CcDd 1/4 CD1/4 Cd1/4 cD1/4 cd 1/4 CD 1/4 Cd 1/4 cD 1/4 cd 1/16 CCDD 1/16 CCDd 1/16 CcDD 1/16 CcDd 1/16 CCDd 1/16 CcDD 1/16 CcDd 1/16 Ccdd 1/16 Ccdd 1/16 CCdd 1/16 CcDd 1/16 CcDd 1/16 ccdd 1/16 ccDD 1/16 ccDd 1/16 ccDd

Pleiotropy Alleles at a single locus may have effects on two or more traits Classic example is the effects of the mutant allele at the beta-globin locus that gives rise to sickle-cell anemia

Genetics of Sickle-Cell Anemia Two alleles 1) Hb A Encodes normal beta-hemoglobin chain 2) Hb S Mutant allele encodes defective chain Hb S homozygotes produce only the defective hemoglobin; suffer from sickle-cell anemia

Sickle-Cell Anemia At low oxygen levels, cells with only Hb S hemoglobin “sickle” and stick together This impedes oxygen delivery and blood flow Over time, it causes damage throughout the body

Campodactyly: Unexpected Phenotypes Effect of allele varies: –Bent fingers on both hands –Bent fingers on one hand –No effect Many factors affect gene expression

Polygenic Traits Result from the combined expression of several genes Skin color, eye color Population may show continuous variation

Continuous Variation A more or less continuous range of small differences in a given trait among individuals The greater the number of genes and environmental factors that affect a trait, the more continuous the variation in versions of that trait