Mendel's Principles of Heredity

Lectures 2-3

two alleles for each trait separate during

gamete formation

two gametes, one from each parent, unite

at random at fertilization

gamete

specialized cells that carry sperm between generations (egg/sperm)

segregation

separation of alleles so that each gamete receives only one copy of each gene

in a monohydric cross, each

F1 hybrid produces two kinds of gametes in a 1:1 ration

F2 progeny

• 3:1 ratio of phenotypes

• ¼ will breed true for the dominant trait

• ½ will be hybrids

• ¼ will breed true for the recessive trait

genotype is a

pair of alleles in an individual (YY or Yy)

phenotype is an

observable characteristic determined by genotype (yellow or green pea seeds_

homozygote

two identical alleles (YY or yy)

heterozygote

two different alleles (Yy)

heterozygote phenotype defines

the dominant allele

unknown genotype

Y- stands for either YY or Yy

probability

a measure of how likely some event is to occur

• expresses a belief in future events, or outcomes

phenomena can occur in multiple ways, but usually in

one way at a time

the outcome of interest usually referred to as an

event

independent events are where

the probability of event 1 does not affect the probability of event 2

probability ranges from

0 (impossible) to 1.0 (certainty)

product rule

probability of two independent events occurring together is the product go their individual probabilities

rule: if two things are independent , you

multiply their probabilities

sum rule

probability of either of two mutually exclusive events occurring is the sum of their individual probabilities m

mutually exclusive events

if one happens, the others can’t

rule: if one thing OR another can happen, you

sum their probabilities

from a cross of Yy x Yy peas, genotypes in F2 progeny are

in 1:2:1 ration (1/4 YY, ½ Yy, ½ yy)

from a cross of Yy x Yy peas, phenotypes in F2 progeny are

3: 1 ratio (3/4 yellow, ¼ green)

a test cross can reveal

an unknown genotype in an individual with a dominant phenotype

testcross solution

test cross with homozygous recessive (yy) and examine progeny

individuals don’t just have

one trait to be inherited

how do two pairs of alleles segregate in an individual that is heterozygous for two traits simultaneously (a dihybrid)?

independent assortment

mendel’s dihybrid crosses revealed the

law of independent assortment

what did Mendel observe for his dihybrid crosses?

the different types of germinal cells of a hybrid are produced on average in equal numbers

a dihybrid cross produces

parental and recombinant types

F1 dihybrids produce

four possible gametes in 1:1:1:1 ratio

four phenotypic classes occurred in the F2 generation

• two are parental

  • two are recombinant

independent assortment in crosses in F1 dihybrids produces

a 9:3:3:3:1 phenotypic ratio

independent assortment

during gamete formation, different pairs of alleles segregate independently of each other

independent assortment - genotype at one locus says

nothing about genotype at a second locus

independent assortment - different types of gametes produces in

equal numbers

2-gene inheritance - if genes assort independently:

• in F2 Yellow: green = 3:1 (3/4 yellow, ¼ green)

• in F2 round:wrinkled = 3:1 (3/4 round, ¼ wrinkled)

dihybrid cross =

2 independent monohybrid crosses

testcross dihybrids to

individuals that are homozygous for both recessive traits

the possible number of gamete genotypes from a hybrid =

2^n → n = number of heterozygote traits

2^n only works if all genes are

heterozygous

mendel’s work:

• explained reappearance of hidden traits

• disproved blendend inheritance

• showed that parents contributed equally to next generation

  • developed a testable model for making predictions about inheritance

a specific gene determines

a specific enzyme, whose activity may affect phenotype

a dominant alleles usually determines

a normally functioning protein

a recessive allele usually does

not encode a functional protein

the R alleles in pea shape encodes

the stitch branching branching enzyme (Sbe1)

• this creates the round shape

the r allele in pea shape does not make

Sb1

• sucrose and unbranched starch build up, leading to wrinkled peas

pea color - the Y allele encode the

stay green enzyme (Sgr)

• Sgr helps break down chlorophyll → resulting in yellow color

pea color - the y allele does

not make Sgr

• chlorophyll is not broken down → pea remains green