Chapter 12 – Patterns of Inheritance What is inheritance? Why study inheritance? What is the relationship between genes, alleles, phenotype and genotype? What is genetic linkage? How can inheritance be predictive? (why do we bother with Punnett squares?

What is inheritance? The genetic transmission of characteristics from parents to offspring

Geneticists study inheritance Genes are inherited

Why study inheritance? Many diseases involve inheritance

Why study inheritance?

Genetics benefits medical advances

What is the relationship between genes, alleles, genotype and phenotype?

Alleles are differences (variation) in a gene within a population of a species Genes determine phenotype (physical characteristic) of a structure, molecule, etc.

Why are there alleles? Point mutations in the wild type gene result in new alleles

Alleles, genotype and phenotype 1.Diploid organisms have 2 alleles for every gene 2.For each gene, the 2 alleles may be identical or they may be different a)Homozygous – possessing 2 identical alleles for a gene b)Heterozygous – possessing 2 different alleles for a gene

What is the relationship between genes, alleles, genotype and phenotype? 1.The type of interaction between the pair of inherited alleles for a gene determines the phenotype a)Dominant allele – expressed, regardless of the other allele present b)Recessive allele – not expressed in the presence of a dominant allel

What is a genotype? A description of the alleles present for a gene 1.Homozygous dominant 2.Homozygous recessive 3.Heterozygous

How is genotype symbolized? Capital letter = dominant allele Lower case letter = recessive allele

What is the relationship between genes, alleles, genotype and phenotype? Tongue rolling gene has 2 different alleles Rr rr RR

What is the relationship between genes, alleles, genotype and phenotype? Not all genes/alleles display dominant/recessive interactions 1.Codominance – both alleles for a gene are expressed (example: two forms of an enzyme) 2.Incomplete dominance – heterozygous phenotype is different from either the dominant or recessive phenotype 3.Many traits influenced by more than one gene

Red flowers and thorns are dominant to white flowers and no thorns in roses. What would be the phenotype of a rose heterozygous for both traits? 1.RrTt 2.Red, thorny 3.RRTT 4.White, thornless 5.rrtt

Red flowers and thorns are dominant to white flowers and no thorns in roses. Which of the following can be a genotype for a red, thorny rose? 1.RT 2.RRTT 3.RrTT 4.RrTt 5.2, 3 & 4

What is genetic linkage? Genes located close together on the same chromosome are genetically linked (i.e they tend to get inherited together)

Mapping is based on genetic linkage Mapping – identifying the location of a gene on a chromosome Some genes are more likely to stay together (during meiosis), other genes are not

Genetic linkage vs. independent assortment Which genes are most likely to be inherited together? a2a 3a 4a 5a 6a7a

1)2 & 2a 2)1 & 5 3)1a&2a 4)5&7 5)1a & 4a 6)Responses 3, 4 & 5 are all correct a 2a 3a 4a 5a 6a 7a

Independent assortment of chromosomes “shuffles” maternally inherited and paternally inherited genes during meiosis = genes materally inherited (maternal alleles ) 1a-7a = genes paternally inherited (paternal alleles ) 5a 6a 7a 1a 2a 3a 4a a 2a 3a 4a a 6a 7a

Crossing over breaks genetic linkage 1a 2a 3a 4a a a 3a 4a

How can inheritance be predictive? How can one know if two genes are located on the same chromosome or on different chromosomes? A A B B A A B B

Mapping genes and genetic crosses Cross individuals different in two genetic traits: AABB x aabb

II. The foundation of genetics A. Gregor Mendel (1860s)

PP pp Pp genotypes (phenotype)

Pp x Pp P pPp Possible gametes

P p P p Homologous pairs are separated in meiosis meiosis

Pp x Pp P pPp PPPpPppp Possible gametes

Geneticists utilize dihybrid crosses Dihybrid crosses involve two genes AABB x aabb A A B B Assuming independent assortment

Genetic linkage A A B B A A B B or

Are the two genes for eye color located on the same chromosome? AABB x aabb F 1 : all AaBb Self the F 1 AaBb x AaBb What types of gametes are produced?

Gametes, assuming independent assortment a A b B a A B b a A B b or Ab aB AB ab AaBb

Gametes, assuming independent assortment a A b B a A B b a A B b or Ab aB AB ab AaBb

Independent assortment

Gametes, assuming complete linkage a A B b AaBb a b A B AB ab

Complete linkage AaBb x AaBb AB AB ab ab AABB AaBb aabb a A B b

Genetic linkage can be broken Crossing over can break linkage Gametes produced: AB Ab aB ab

Gametes, assuming independent assortment a A b B a A B b a A B b or Ab aB AB ab AaBb 25%

Gamete frequency – linked genes a A B b No crossing over AB ab With crossing over AB ab Ab aB 50% 40% 10%

Linkage between genes varies with distance between genes A BCD a bcd

Linkage vs. independent assortment For two single gene traits, if perform a dihybrid cross of heterozygotes: Independent assortment gives 4 phenotypes in offspring Complete linkage gives <4 phenotypes Partial linkage gives 4 phenotypes in a different ratio than independent assortment

Genetics & disease Most disease genes are autosomal & recessive (Most mutations function as recessive alleles to “normal” functioning alleles) Each person is a “carrier” of several recessive disease related alleles

Sex-linked traits Genes located on X-chromosome Males possess only 1 allele for sex-linked traits