GENETICS

I. GREGOR MENDEL (1822-1884)

General

Monk

Worked with pea plants

Self-fertilization

Simple traits

Used Statistics

Looked at 1 trait at a time

A. Experiment – example

Parents purple flower x white flower

F1 100% purple

F2 75% purple 25% white

WHAT HAPPENED?

II. MENDELIAN GENETICSA. . DEFINITIONS

Gene – a segment of DNA that codes for a specific trait. It is found at a specific locus (location) on a chromosome

Chromosome – A condensed structure made of one or two DNA molecules with genes.

Allele – A form of a gene.

2 chromatids 1 chromatid

T or t

Dominant – Allele that is expressed when present.

Recessive – Allele expressed when the dominant is not present.

T

t

Co-dominant – Alleles for the same gene that both express themselves when present.

Incomplete dominance – The heterozygous

genotype results in a phenotype between the two homozygous genotypes.

ABO Blood

RR Rr rr

Red Pink White

Genotype – The alleles an organism/cell has.

Phenotype – The physical expression of a gene.

TT Tt tt

Tall Tall Short

Homozygous – The homologous pair has the same alleles for the same gene.

Heterozygous – The homologous pair has different alleles for the same gene.

TT or tt

Tt

Diagram

Gene – Coat Color

Chromosome

Allele

Dominant

Recessive

Genotype Bb

Phenotype black

B. Monohybrid Cross –

Look at one gene at a time

1. Principle of SegregationEvery individual (diploid) has 2 copies

(alleles) of a gene (one from the female and one from the male) for every trait.

In the production of gametes (by Meiosis) these alleles are separated randomly and come back together in the formation of a zygote (fertilization).

2. Example – Coat Color in Labrador

B = black (dominant) b = brown (recessive)

Phenotype: Black Black Brown

Parent Genotype: BB Bb bb

Parent gametes: B B B b b b

Example of the cross

Male Female

Bb Bb

B b B b gametes

BB Bb

Bb bb

Punnett square

B b

B

b

Genotype

1BB:2Bb:1bb

Phenotype

3black:1brown

C. DIHYBRID CROSS – When look at two genes at a time.

Linked genes are when genes are on the same chromosome.

Example

1. PRINCIPLE OF INDEPENDENT ASSORTMENT

Genes on different chromosomes (so are not linked)

randomly assort into gametes during Meiosis and have an

equal chance for any combination.

Meiosis I or Meiosis I

2. Example Gene Alleles

ear lobes E=free e=attached

freckles F=have f=none

Parent Genotypes EeFf EeFf

Gametes EF Ef eF ef EF Ef eF ef

EACH GAMETE MUST CONTAIN ONE COPY OF EACH GENE (in

this case each must have an E/e and a F/f)

EEFF EEFf EeFF EeFf

EEFf EEff EeFf Eeff

EeFF EeFf eeFF eeFf

EeFf Eeff eeFf eeff

EF Ef eF ef

EF

Ef

eF

ef

Free earlobes, freckles

Free earlobe, none

Attached, freckles

Attached, none

SEX-LINKED

• Gene is on the X chromosome (usually).

Male XY

Ex. Recessive trait - XHY or XhY

Female XX

Ex. Recessive trait – XHXH or XHXh or XhXh

Co-dominant

When more than one allele for any given gene are

both dominant and express themselves if present.

Example:ABO Blood

Phenotype Genotype

A blood IAIA or IAiB blood IBIB or IBi

AB blood IAIB

O blood ii

ABO are sugar antigens on the surface of

the red blood cells (rbc)

Rh factors are proteins on the surface of rbc.

Incomplete dominance

The heterozygote has a phenotype between

the two homozygous genotypes

GENOTYPES RR RR’R’R’

PHENOTYPES red pink white

III. PedigreeA tool used to track a genetic history. Used

for a family for some specific trait.

without trait with trait

Female

Male

mated

offspring

GENETIC PROBLEMS

MONOHYBRIDIn silkworms yellow cocoon is dominant to white cocoon.

Two silkworms are mated (one heterozygous and the

other homozygous recessive).

Find the: parent genotype and phenotypegametes producedPunnett SquareOffspring genotypeOffspring phenotype

Albinism is inherited as a recessive trait.

a. A heterozygous female has children by a homozygous normal male. What % of their offspring will be carriers for albinism?

b. A normally pigmented man whose mother was albino marries a homozygous normal woman. What % of their offspring will be homozygous normal?

c. A heterozygous male marries an albino female. What percentage of their offspring will be albino?

DIHYBRIDIn hogs, a gene that produces a white belt around the

animal’s body is dominant over its allele for a uniformly

colored body. Another independent gene produces fusion

of the two hooves on each foot (an instance of syndactyly).

It is dominant over its allele, which produces normal

hooves.

Supposed a uniformly colored male hog homozygous for syndactyly is mated with anormal-footed female hog homozygous for thebelted character.

CO-DOMINANCEIn cattle, red is not dominant to white. When the

two colors are crossed, the claves have both red

and white hairs, called roan.

A rancher mates a red bull and white cow.

Show Punnett Square and label phenotypes.

INCOMPLETE DOMINANCEIn four o’ clock flowers there are red, white and

pink petal colors. Pink petals is a heterozygous condition resulting from a cross of red and white.

Cross two pink plants

BLOOD TYPE AND SEX-LINKED

A normal visioned female has type A blood.

Her father was colorblind with type O blood

and her mother had AB blood. She marries

a man who is colorblind and has type B

blood (whose mother had normal vision and

O blood and whose father had normal vision

and B blood).