biology 3201 unit 3 genetics

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Biology 3201 Unit 3 Genetics #1Mendelian Genetics - Introduction (Ch 16)

Mendelian genetics

✤ Genetics: the scientific study of heredity

✤ Heredity: the passing of traits from one generation to the

next

✤ Trait: any observable characteristic that an organism may

have

Ex. Eye colour, hair colour, height, tongue rolling

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Gregor Mendel

✤ An Austrian monk who studied genetics by

growing and cross breeding pea plants

✤ Why pea plants?

They reproduced quickly

Their traits were easily observable

They were readily available

They could self pollinate meaning he

could control which plants pollinated

Mendel's 1st experiment (part one)

✤ Took the seeds of a purebred tall plant and the seeds

of a purebred short plant and cross-pollinated them

and collected the seeds produced by this cross and

planted them

✤ Results: all of the offspring were tall (no short plants)

✤ So did the trait for short disappear??

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Mendel's 1st experiment (part two)

✤ Next, Mendel self pollinated the plants from the F1 generation.

These seeds were planted and grown.

✤ Results: Three quarters were tall and one quarter was short.

✤ To describe his findings Mendel said there was a dominant

factor at play causing the plants to be tall in the first cross and

that a recessive factor was being expressed in the second cross.

✤ Principle of Dominance: when an organism is crossed for a pair

of contrasting traits ONLY the dominant trait can be seen in the

hybrid. The recessive factor was hidden.

Mendelian Genetics Terminology

✤ Unit factors: these were the two factors that Mendel said affected the

expression of traits in organisms. Each organism has two factors for each

trait. We now call them GENES.

✤ Unit Theory of Inheritance: this was Mendel's theory where he believed

unit factors (now called genes) control the expression of traits in offspring

✤ Dominant factor: this was a factor that was ALWAYS expressed in an

organism. Represented by an uppercase (capital) letter in a Punnett

square

✤ Recessive factor: this was a factor that SOMETIMES was expressed in an

organism. Represented by a lowercase letter in a Punnett square

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Mendelian terminology cont...

✤ Allele: this is the form a gene can be. It can be

dominant or recessive but not both.

✤ F1 generation: also called First Filial Generation. This

is the first set of offspring that are produced by parents.

✤ F2 generation: also called the Second Filial

Generation. This is the second set of offspring created

from the offspring of the parents. These offspring are

the same thing as grandchildren.


✤ Pure breed/strain: a term that refers to an organism having

EITHER two dominant factors OR two recessive factors for a trait

Ex. A plant that is pure tall have two dominant factors for

height- one from the mother and one from the father

Ex. A plant that is pure short has two recessive factors for

height - one from the mother and one from the father

✤ Hybrid: a term meaning that an organism has one dominant and

one recessive factor for a trait

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✤ Homozygous: a term meaning that an organism has the SAME

alleles for a gene (both are dominant or both are recessive). Same

as pure breed

✤ Heterozygous: a term meaning an organism has DIFFERENT alleles

for a gene (one is dominant and one is recessive). Same as hybrid

✤ Genotype: the genetic makeup of an organism. It is combination of

alleles for an organism.

✤ Phenotype: the physical characteristics displayed by an organism. It

is the appearance of a trait in an organism.


✤ Punnet square: a square

used to help determine

the possible/probable

outcomes of a cross

between two individuals

✤ Cross: another way of

saying that a female has

been mated with a male

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Mendel's Laws

✤ 1. Principle of Dominance: a principle proposed by Mendel that

stated that the dominant form of a gene (trait) will ALWAYS be

expressed

✤ 2. Law of Segregation: a law proposed by Mendel that stated that

a pair of factors for a trait separated (segregated) during the

formation of gametes (sperm and egg) and then recombined

during fertilization

✤ 3. Law of Independent Assortment: a law stating that during

meiosis, genes for different traits are separated and distributed to

gametes independently of one another.

The single factor cross

✤ Single factor cross: a cross that is

done for ONE TRAIT.

✤ Ex. Straight hair is dominant to curly

hair. Mary is homozygous for

straight hair (HH), while Patrick is

heterozygous for straight hair (Hh).

What are the possible genotypic and

phenotypic ratios of a cross between

these two individuals?

✤ Genotypic ratio: 1/2 HH: 1/2 Hh

✤ Phenotypic ratio: all straight hair

H H

H HH HH

h Hh Hh

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The Monohybrid Cross

✤ Monohybrid cross: a single factor cross where the

parents are HETEROZYGOUS (HYBRID) for the trait

in question.

✤ Ex. In pea plants, tall (T) is dominant to short (t).

What would be the genotypic and phenotypic ratios of

a cross between two heterozygous pea plants?

The Monohybrid Cross

✤ Genotypic ratio: 1/4 TT:1/2 Tt:

1/4 Tt

✤ Phenotypic ratio: 3 tall: 1 short

✤ Note: In a MONOHYBRID

CROSS, these ratios are

ALWAYS the same.

✤ Genotypic: 1:2:1

✤ Phenotypic: 3:1

T t

T TT Tt

t Tt tt

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The product rule

✤ Product rule: the probability, or chance, that two or more

independent events will occur together is the product of

their individual probabilities occurring alone.

✤ Ex. What are the chances of a couple have 3 girls in a

row?

✤ Chance of one girl =1/2

✤ So for three in a row it would be 1/2 x 1/2 x 1/2 = 1/8

Test cross

✤ Test cross: a cross in which an individual of an unknown genotype is

crossed with a homozygous recessive individual

✤ This is done to determine the genotype of the unknown individual.

✤ Two crosses are performed (on paper) before mating

Cross a homozygous dominant individual with a homozygous recessive

individual

Cross a heterozygous individual with a homozygous recessive individual

✤ Compare results of mating with crosses to identify unknown individual

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Test cross- Homozygous Dominant

✤ Parents: TT x Tt

✤ Genotypic ratio: 100%

Tt

✤ Phenotypic ratio: all

tall

T T

t Tt Tt

t Tt Tt

Test cross -Heterozygous

✤ Parents: Tt x tt

✤ Genotypic ratio: 1/2 Tt:

1/2 Tt

✤ Phenotypic ratio: 1/2 tall:

1/2 short

T t

t Tt Tt

t tt tt

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Two factor crosses

✤ Two factor cross: a cross performed where two traits are involved

at the same time

✤ Ex. A cow that is homozygous dominant for brown coat (BB) and

homozygous dominant for split hoof (HH) is crossed with a cow

that is homozygous recessive for brown hair (bb) and

homozygous recessive for non-split hoof (hh). What are the

possible genotypic and phenotypic ratios for the offspring?

✤ Parents: BBHH x bbhh

✤ Gametes: BH, BH, BH, BH and bh, bh, bh, bh

Two factor crosses (cont)

BH BH BH BH

bh BbHh BbHh BbHh BbHh




Genotypic ratio: 100 % BbHh

Phenotypic ratio: all black coat with split hoof

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Dihybrid cross

✤ Dihybrid cross: a two factor cross where both parents are

HETEROZYGOUS for BOTH traits

✤ Ex. In cows, the gene for black coat colour (B) is dominant to the

gene for white coat colour (b). The gene for split hoof (H) is

dominant to the gene for solid hoof (h). What are the expected

genotypic and phenotypic ratios for a dihybrid cross between two

cows?

✤ Parents: BbHh x BbHh

✤ Gametes: BH, Bh, bH, bh and BH, Bh, bH, bh

Dihybrid cross

BH Bh bH bh

BH BBHH BBHh BbHH BbHh

Bh BBHh BBhh BbHh Bbhh

bH BbHH BbHh bbHH bbHh

bh BbHh Bbhh bbHh bbhh

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Dihybrid cross

✤ Genotypic ratio: 1/16 BBHH: 2/16 BBHh: 2/16 BbHH: 4/16 BbHh: 1/16

BBhh: 2/16 Bbhh: 1/16 bbHH: 2/16 bbHh: 1/16 bbhh

✤ Phenotypic ratio:

9 black coat/split hoof

3 black coat/solid hoof

3 white coat/split hoof

1 white coat/solid hoof

✤ Note: these ratios remain constant for any dihybrid cross.

biology 3201 unit 3 genetics

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