Mendel, Genes, and Inheritance Chapter 12

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

Mendel’s Work Bred pea plants  cross-pollinated true breeding parents (P)  raised seed & then observed traits (F 1 ) filial  allowed offspring to cross-pollinate & observed next generation (F 2 )

Mendel’s Data:

Mendel’s Experiments true-breeding purple-flower peas true-breeding white-flower peas X 100% purple-flower peas F 1 generation (hybrids) 25% white-flower peas F 2 generation 75% purple-flower peas 3:1 P self-pollinate

Genes Sequences of DNA that contain information about specific traits, by coding for individual proteins Passed from parents to offspring Each has a specific location (locus) on a chromosome Alleles Different versions of the same trait

Genes and inheritence For each characteristic (gene), a diploid organism inherits 2 sets of alleles  1 from each parent Homologous chromosomes  Pairs of chromosomes that contain the same traits (genes)  For each trait there are two alleles (one on each homologous chromosome)

Genetic Terms purple-flower allele & white-flower allele are 2 DNA variations at flower-color locus different versions of gene on homologous chromosomes

Mendel’s Findings: Dominance Some alleles for a given traits mask others Dominant allele  Fully expressed in a hybrid  Designated by a capital letter e.g. P = Purple allele Recessive allele  no noticeable effect in a hybrid  Designated by a lowercase letter E.g. p = White allele

Genotype vs. phenotype Phenotype  Description of an organism’s trait  “visible” characteristic Genotype  Description of an organism’s genetic makeup; i.e. which alleles are present in the organism  Alleles may be the same or they may be different: Homozygous = same alleles; PP, pp Heterozygous = different alleles; Pp

Looking closer at Mendel’s work true-breeding purple-flower peas true-breeding white-flower peas X 100% purple-flower peas 25% white-flower peas 75% purple-flower peas 3:1 PPpp PpPpPpPpPpPpPpPp self-pollinate ???? Phenotype: 100% purple-flower peas F 1 generation (hybrids) F 2 generation 75% purple-flower peas 3:1 P

Punnett squares Pp x Pp Pp male / sperm P p female / eggs PP PpPppp PpPp PPPpPpPpPp pp 75% 25% 3:1 25% 50% 25% 1:2:1 % genotype % phenotype

Phenotype vs. genotype 2 organisms can have the same phenotype but have different genotypes PP homozygous dominant purplePpPp heterozygous purple

Dominant phenotypes It is not possible to determine the genotype of an organism with a dominant phenotype by looking at it. So how can you figure it out? PP? Pp?

Test cross Cross-breed the dominant phenotype — unknown genotype — with a homozygous recessive (pp) to determine the identity of the unknown allele x pp is it PP or Pp?

x PPpp Test cross pp P P PpPp PpPpPpPp PpPp pp P p PpPp pp PpPp x PpPp 100% 50%:50% 1:1

Mendel’s laws of heredity (#1) PP P P pp p p PpPp P p Law of segregation  when gametes are produced during meiosis, homologous chromosomes separate from each other  each allele for a trait is packaged into a separate gamete

Law of Segregation What meiotic event creates the law of segregation? Meiosis 1

Monohybrid cross Some of Mendel’s experiments followed the inheritance of single characters  flower color  seed color  monohybrid crosses

Dihybrid cross Other of Mendel’s experiments followed the inheritance of 2 different characters  seed color and seed shape Dihybrid crosses

Dihybrid cross true-breeding yellow, round peas true-breeding green, wrinkled peas x YYRRyyrr P YyRr 100% F 1 generation (hybrids) yellow, round peas self-pollinate F 2 generation 9/16 yellow round peas 9:3:3:1 3/16 green round peas 3/16 yellow wrinkled peas 1/16 green wrinkled peas Y = yellow R = round y = green r = wrinkled

Dihybrid Cross How are the alleles on different chromosomes handed out?  together or separately? YyRr YRyr YyRr YryRYRyr

Dihybrid cross YyRr YRYryR yr YR Yr yR yr YYRRYYRrYyRRYyRr YYRrYYrrYyRrYyrr YyRRYyRryyRRyyRr YyRrYyrryyRryyrr x 9/16 yellow round 3/16 green round 3/16 yellow wrinkled 1/16 green wrinkled

Mendel’s laws of heredity (#2) Law of independent assortment: each pair of alleles segregates into gametes independently  4 classes of gametes are produced in equal amounts YR, Yr, yR, yr  only true for genes on separate chromosomes YyRr Yr yR YR yr

Law of Independent Assortment What meiotic event creates the law of independent assortment? Meiosis 1

The chromosomal basis of Mendel’s laws… Trace the genetic events through meiosis, gamete formation & fertilization to offspring

Review: Mendel’s laws of heredity Law of segregation  each allele segregates into separate gametes Law of independent assortment  Observable in dihybrid (or more) cross 2 or more traits  each pair of alleles for genes on separate chromosomes segregates into gametes independently Each gamete carries one allele of each trait

Extending Mendelian genetics Mendel worked with a simple system  peas are genetically simple  most traits are controlled by a single gene  each gene has only 2 alleles, 1 of which is completely dominant to the other The relationship between genotype & phenotype is rarely that simple

Incomplete dominance Heterozygotes show an intermediate phenotype  RR = red flowers  rr = white flowers  Rr = pink flowers make 50% less color

Incomplete dominance true-breeding red flowers true-breeding white flowers X 100% 100% pink flowers F 1 generation (hybrids) 25% white F 2 generation 25% red 1:2:1 P self-pollinate 50% pink

Incomplete dominance C R C W x C R C W CRCR CWCW male / sperm CRCR CWCW female / eggs CRCRCRCR CRCWCRCW CWCWCWCW CRCWCRCW 25% 1:2:1 25% 50% 25% 1:2:1 % genotype % phenotype CRCRCRCR CRCWCRCW CRCWCRCW CWCWCWCW 25% 50%

Codominance BOTH alleles express in heterozygotes Example: In chickens, feather color trait has two alleles: B= Black feathers W = White feathers  What is phenotype of BB, WW?  What is the phenotype of BW?

Codominance: WWBB WB +

Multiple Alleles More than two possible alleles for one trait Example: in ABO blood group, 3 possible alleles:  I A, I B, i 4 blood types: A, B, O, AB How?

Pleiotropy Most genes are pleiotropic  one gene affects more than one phenotypic character wide-ranging effects due to a single gene: dwarfism (achondroplasia) gigantism (acromegaly)

Epistasis One gene masks another  coat color in mice = 2 genes pigment (C) or no pigment (c) more pigment (black=B) or less (brown=b) cc = albino, no matter B allele 9:3:3:1 becomes 9:3:4

Epistasis in Labrador retrievers 2 genes: E & B  pigment (E) or no pigment (e)  how dark pigment will be: black (B) to brown (b)

Polygenic inheritance (continuous variation) Some phenotypes determined by additive effects of 2 or more genes on a single character  phenotypes on a continuum  human traits skin color height weight eye color intelligence behaviors

Environmental influence The expression of some genes can be influenced by the environment for example: coat color in Himalayan rabbits and Siamese cats  an allele produces an enzyme that allows pigment production only at temperatures below 30 o C

Environmental influence

Acknowledgement Much of this presentation was modified from the Biology Zone website: