basic principles of genetics university of utah review of genes etc! genetics from the beginning!
TRANSCRIPT
Basic principles of genetics
University of Utah Review of Genes etc!
Genetics from the beginning!
The studyof
Heredity
Inheritance of traits
coded for bygenes
What is a gene pool? All the genesin a given population
Evidence for Heredity
Similar phenotypecaused by similarities
in genotype
Gregor MendelAustrian Monk!
Mendel’sPea
PlantsShowed Traits
that wereMarkedly
Contrasting Could tell tall from short easily
Traits studied by Mendel
Markedly contrasting
Female
part with
egg
Male part with pollen
Cross Pollination/Cross Fertilization
Pollen from anther transferred to stigma of female part
Gregor Mendel• Cross pollinated pea plants• Studied traits whose expression
were markedly contrasting• eg Height: tall vs. short…..no in
between• Pure Tall X Pure Short…….cross 1• All offspring Tall
• He then crossed offspring from cross 1
• Offspring: 75% Tall : 25% Short
• He reasoned that……
•Gene for tall is dominant
•Gene for short is recessive
Summary of Mendel’s Cross
• TT x tt --------Tt
• Tt x Tt -------- 3 Tall : 1 Short
• Parents referred to as P1• First generation referred to as F1
(F=Filial)
• Second generation referred to as F2
T T
t
t
Tt Tt
Tt Tt
Monohybrid Cross: Involving 1 trait
PhenotypesAll Tall
GenotypesAll Tt
Heterozygous
Punnett Square
T t
T
t
TT Tt
Tt tt
Monohybrid Cross
Phenotypes3Tall: 1Short
Genotypes1 TT : 2 Tt : 1 tt
Genotypes of parents
F1
F2
Punnett Square
Which characteristic appears to be recessive?
Crossing Tall and Short Plants
Genetics Terminology• Physical appearance = phenotype
• Genetic make up causing pheno = genotype
• Genotype described using minimum of 2 letters
• All traits controlled by at least 2 genes
• These referred to as alleles of a pair
• For height: alleles are T and t
• If genotype is TT, plant is tall (homozygous dominant)
• If genotype is Tt, plant is also tall but also a carrier for short (heterozygous)
No need to
say
Heterozygous d
ominant…why??
Hybrid & Heterozygousoften confused
More terminology…Yikes!!.• If genotype is tt, plant is short (homozygous
recessive or just recessive)
• Phenotypic ratio in cross 1 = all tall
• Phenotypic ratio in cross 2 = 3 tall : 1 short
• Genotypic ratio in cross 2 = 1 TT: 2 Tt: 1 tt
• G.R. & P.R. found easily using punnett square
Identify the recessive allele & phenotypeUsing the following punnett square
Monohybrid Cross
Phenotypes1 Tall: 1 Short
Genotypes1 Tt : 1 tt
t t
T
t
Tt Tt
tt tt
Conclusions from Mendel’s work
• Inherited characteristics controlled by factors (genes) found in gametes
• All traits controlled by 2 alleles (minimum).
– Some alleles mask effect of others.
• These are called dominant alleles
• Law of Segregation: Pairs of genes segregate when homologous chromosomes move to opposite poles during meiosis
• Law of Independent Assortment: Different pairs of alleles segregate independently of each other
Mendel’s law of segregation
Test Cross• Used to determine genotype of individual
expressing dominant characteristic
• eg. If pea plant is tall
• Genotype could be TT or Tt
• Only one way to find out!
• Cross with homozygous recessive
Test Cross
PhenotypesAll Tall
GenotypesAll Tt
t t
T
T
Tt Tt
Tt Tt
No shortoffspring
Test Cross
Phenotypes2 Tall : 2 short
Genotypes2 Tt : 2 tt
t t
T
t
Tt Tt
tt tt
Shortoffspring
Summary of Test Cross Results
• If some recessive individuals show up…..
• Genotype of unknown must be heterozygous
• If no recessives show up….
• Genotype is probably homozygous dominant
Testcross with monohybrids
Test Crossing Sheep
• Sheep ranchers like producing white wool
• Easier to dye and less brittle than black
• White is dominant, black recessive
• If farmer suspects white sheep in herd are heterozygous…what can she/he do????
• Farmer may find out for certain by crossing suspected carriers with homozygous recessive (ww)
• If some black sheep show up as offspring…
• Sheep was a carrier for black wool! (heterozygous)
• These sheep make better meat!!!!