19-Genetics (F13) Part I

GENETICS

Chapter 14

Genetics & Inheritance

Genetics – study of inheritance (heredity), how traits are passed from one generation to the next

Patterns of Inheritance – thoughts in the 1800s: Blending Inheritance Inheritance of Acquired Characteristics

Inheritance Hypotheses (1800s)

Inheritance of acquired characteristics

“pangenes” from organs passed on to homonculus

Jean-Baptiste Lamarck

Homunculus

http://en.wikipedia.org/wiki/Image:Jean-Baptiste_Lamarck.jpg

Gregor Mendel

Austrian monk

Studied genetics using garden pea plants

Mathematician – applied statistics and laws of probability to his experiments

Developed a particulate theory of inheritance

Gregor Mendel – father of modern genetics

Garden PeasFirst Model Organism in Genetics

Easy (and inexpensive) to grow

Produce many offspring

Short reproductive cycle

Able to control matings

Easily recognizable traits

7 different traits

2 options for each trait

Garden PeasControlling Mating

Self-fertilization (self-pollination) – pollen grains (contain sperm) from one flower are used to fertilize the carpel (contains eggs) of the same flower

Carpel (female organ) receives pollen

Eggs

Stamen (male organs) produce pollen grains

Cross-pollination – pollen from the flower of one plant fertilizes the carpel on a flower of a different plant

3. Transfer pollen to the female organs of the individual whose male organs have been removed.

2. Collect pollen from a different individual.

1. Remove male organs from one individual.

?

Garden PeasControlling Mating

Mendel’s Experiments

Started with true-breeding plants; when self-crossed, every generation has same phenotype

Experiment 1:Crossed two true-breeding plants that had alternate phenotypes – recorded phenotypes of offspring (F1)

Experiment 2:Crossed F1 plants with one another – recorded phenotypes of offspring (F2)


Experiment #1:

true-breeding purple x true-breeding white


Experiment #1: Results

What happened to the “white” trait? Missing? Masked?


Experiment #2: A Monohybrid Cross

Self-fertilize F1 generation plants(offspring from Experiment 1)


Experiment #2: Results

3:1 ratio

Conclusion: Genetic information for

“white” was only masked

Dominant and Recessive Traits

Genetic information that codes for a particular trait can be:

Dominant – masks the trait of recessive genetic information paired with it (A)

Recessive – trait is fully or partially masked by dominant genetic information (a)

Mendel’s ConclusionsParticulate Inheritance Hypothesis

Parents transmit information about traits to their offspring as hereditary “factors” (particles) hereditary “factors” do not blend together or acquire new

or modified characteristics through use, rather… hereditary “factors” maintain their integrity from

generation to generation

Hereditary “factors” inherited from parents determine traits observed in offspring


3:1

See 3:1 ratio with

all of these traits

Mendel’s ConclusionsPrinciple of Segregation

A 3:1 ratio is possible if…

Each parent has two hereditary “factors” for each trait

The factors separate (segregate) during the formation of gametes

Each gamete contains only one factor for a particular trait

Fertilization (random) gives each new individual two factors for each trait

How do we put this into modern genetic terms? Let’s review…

Genetics TerminologyReview

Genes – units of genetic information (DNA) about specific traits; hereditary “factors” Passed from parents to offspring Carry directions for the synthesis of polypeptides/proteins

Alleles – alternate versions of a gene, code for different forms of the same trait

Homologous chromosomes – pair of chromosomes have the same size and shape carry the same genes (alleles may differ)

Gene locus – location of a gene on a chromosome

Genetics TerminologyReview

A pair of homologous chromosomes

A gene locus

Alternate versions of a gene (alleles)

B b

Alleles

What is the source of new alleles?

Mutations – heritable changes in the molecular structure of DNA• Original source of all new alleles• source of diversity in life• alter gene (DNA) alter protein alter trait

Alleles

All individuals have two alleles at each gene locus; one inherited from each parent

The alleles can be the same – homozygous for that gene

The alleles can be different – heterozygous for that gene

A pair of homologous chromosomes

Homozygous (identical alleles)

Heterozygous (different alleles)

B b

A A

Alleles

If the two alleles of a gene differ, the one that is observable is called the dominant allele (A), the one that is masked is called the recessive allele (a)

Dominant – allele that masks the trait of any recessive allele paired with it (A)

Recessive – allele whose trait is fully or partially masked by a dominant allele (a)

Genetics Terminology

Genotype – the set of alleles an individual receives at fertilization

Phenotype – an individual’s physical appearance (observable traits)

genotype determines phenotype

Genotypes and Phenotypes

Possible genotypes & phenotypes for one gene:

Genotypes Phenotypes

Homozygous Recessive

aa Recessive trait

Heterozygous Aa Dominant trait

Homozygous Dominant

AA Dominant trait

Mendel’s ConclusionsParticulate Inheritance Hypothesis - Modified

Parents transmit information about traits to their offspring as particular forms of hereditary “factors”- alleles alleles do not blend together alleles maintain their integrity from generation to

generation

Alleles inherited from parents determine phenotype

Sexual Reproduction

Aa fertilization produces

heterozygous offspring

A A A Aa

aa

a

meiosis I

A A A A a a a a

meiosis II

gametes

AA

AAA

aa

aaaa

chromosomes duplicated before

meiosis

Homozygous dominant parent

Homozygous recessive parent

A

Two alleles for gene A

Genotype: aaPrinciple of Segregation

Each gamete contains only one allele for each geneFertilization gives new

individual two alleles for each gene

(one from each parent)

Punnett Square Analysis

Method used to determine possible genotypes (and phenotypes) of offspring

Steps: Determine the genotypes of all possible gametes

donated by mother (eggs) Determine the genotypes of all possible gametes

donated by father (sperm) Determine all possible combinations of gametes - this

represents all possible genotypes of offspring

Mendel’s ResultsExplained Using the Principle of Segregation

Experiment 1:

true-breeding purple x true-breeding white

What are the genotypes of these

flowers?


Experiment 1:

homozygous purple x homozygous whitePP x pp

Results: All offspring are purple


Experiment 1:Parents have two

copies of each gene (diploid)

Gametes have one copy of each gene

(haploid)

Offspring have two copies of each gene

(diploid)


Experiment 2: Monohybrid Cross• Examine pattern of inheritance of alleles at one gene• Cross individuals that are heterozygous (hybrid) at one

(mono) gene

heterozygous purple x heterozygous purplePp x Pp

Results: 3 purple:1 white


Experiment 2: Monohybrid Cross

Mendel’s ResultsReview of the Principle of Segregation

Principle of Segregation:

1. Each individual has two alleles of each gene – located on pairs of homologous chromosomes

2. The alleles segregate (separate) during gamete formation (meiosis)

meiosis

meiosis



3. Each gamete contains only one allele for each trait

4. Fertilization gives each new individual two alleles for each gene (one from each parent)

meiosis

meiosis



3. Each gamete contains only one allele for each trait

4. Fertilization gives each new individual two alleles for each gene (one from each parent)

fertilization

Monohybrid Cross

Aa x Aa Phenotypic ratio = 3:1

3/4 dominant phenotype 1/4 recessive phenotype

Genotypic ratio = 1:2:1 1/4 homozygous dominant 2/4 heterozygous 1/4 homozygous recessive

Rr

Rr

R r

R

r

RR Rr

Rr rr

Determining Probability

What is the probability that parents that are heterozygous for the freckles gene will have a child that does NOT have freckles?

F = frecklesf = no freckles


What is the probability that they will have a child that DOES have freckles?

sum rule – when the same event can occur in more than one way (ex. FF, Ff, and Ff), add results


What is the probability that they will have two children without freckles?

product rule – chance of two independent events occurring together is the product of their chance of occurring separately


How are alleles of different genes (located on different chromosomes transmitted to gametes? Together - dependent assortment or… Separately - independent assortment

T

Tg gt

GG

t

Do the maternal alleles (tg) and paternal alleles (TG) always stay together during segregation?

or…

Do you find a mix of maternal and paternal alleles in the gametes?


How are alleles of different genes (located on different chromosomes transmitted to gametes? Together - dependent assortment or… Separately - independent assortment

Experiment: Dihybrid Cross Look at pattern of inheritance of two genes Cross individuals that are heterozygous (hybrid) at

two (di) genes

Generate dihybrid individual by crossing individuals that are homozygous for two traits

Cross dihybrid individuals…

Dihybrid Cross

F1 offspring all TtGg

Dihybrid Individual

Parents have two copies of each gene

(diploid)

Gametes have one copy of each gene

(haploid)

Offspring have two copies of each gene

(diploid)

If Transmission is Dependent…

T and G stay together in gametes;

t and g stay together in gametes

Phenotypic Ratio

3:1

If Transmission is Independent…

Phenotypic Ratio

9:3:3:1

Alleles at T gene and G gene segregate to gametes independently of each other

Dihybrid Cross (Independent Assortment)

AaBb x AaBb Phenotypic ratio = 9:3:3:1 Genotypic ratio = 1:2:1:2:4:2:1:2:1

Mendel’s Principle of Independent Assortment

Genes located on one chromosome are sorted for distribution into gametes independently of genes located on another chromosome

Maternal alleles (tg) do not always stay together;

paternal alleles (TG) do not always stay together

Why is this the case? Think about the steps in meiosis!

T

Tg gt

GG

t

Metaphase I of Meiosis

Metaphase I Independent Assortment – homologous chromosomes line

up at metaphase plate randomly (and will therefore be distributed into daughter nuclei randomly)

Meiosis I

Replicated chromosomesprior to meiosis

Gam

etes

Alleles for seed shape

Meiosis II

Principle of independent assortment: The genes for seed shape and seed colorassort independently, because they are located on different chromosomes.

Meiosis II

Meiosis I

Alleles for seed color

1/4 TG 1/4 tg 1/4 Tg 1/4 tG

Chromosomes can line up in two ways

during meiosis I

T

Tg gt

GG

T T

T T

T T

t t

t t

t t

t

G G

G G

G G

g g

g g

g gg g

g g

g g

T T

T T

T T

t t

t t

t t

G G

G G

GG



Genes located on one chromosome are sorted for distribution into gametes independently of genes located on another chromosome

Each pair of alleles assorts independently of the other pairs

All possible combinations of alleles can occur in the gametes

Calculating Allele Combinations

What are the possible allele combinations that can occur in the offspring from the following crosses? AaBb x AaBb AABB x AaBb

AaBbDd x AaBbDd

Using a Punnett square when considering crosses of 3 or more traits is cumbersome!

Calculating Allele Combinations

Using a Punnett square when considering crosses of 3 or more traits is cumbersome!

Consider another way… Determine the fraction of each individual set of

alleles like in a monohybrid cross. The probability of a particular combination of alleles

is equal to the product (multiplication) of the individual alleles.

Apply Your Knowledge

What fraction of the offspring of parents, each with the genotype KkLlMm, will be kkllmm? Fraction of kk? Fraction of ll? Fraction of mm?

Suppose two DdEeFfGgHh individuals are mated. What would be the predicted frequency of ddEEFfggHh offspring from such a mating?

1/4 1/4 1/4 1/64x x =

An organism has a dominant phenotype…Example: purple flowers

What is the genotype of the organism?

Need to perform a testcross!

Testcrosses

Testcrosses

Testcross – method used to determine the genotype of an organism that is phenotypically dominant

cross dominant organism with homozygous recessive organism

Example: P__ x pp

What results do you expect if the dominant organism is homozygous dominant? Heterozygous?

Two-Trait Testcross

L = long wingsl = vestigal (short) wingsG = gray bodyg = black body

What are the possible genotypes of these flies?

Two-Trait Testcross

Perform a testcross:

G__ L__ x ggll

Two-Trait Testcross


If this fly as the genotype GGLL, what are the genotypes of all of the possible gametes

it can produce?

What are the expected phenotypes of offspring

resulting from aGGLL x ggll cross?

Two-Trait Testcross


If this fly as the genotype GgLl, what are the genotypes of all of the possible gametes it can

produce?

What are the expected phenotypes of offspring

resulting from aGgLl x ggll cross?

19-Genetics (F13) Part I

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