WHAT IS GENETICS?

GENETICS is the study of how traits are passed from parent to offspring in the form of Genes.

HISTORY!Gregor Mendel

Born 1822

Austrian Monk

Examined reproduction of pea plants

Plants reproductive organs

are called FLOWERS

A flower has both male and female parts.

The pea plants Mendel was working with were typically TRUE-BREEDING, meaning they self-pollinated

EX, TALL pea plants would always be pollinated by tall pea plants and produce tall offspring!

WHAT WE KNOW (MENDEL DIDN’T)

Genes – control a heritable feature (characteristic);

Example: Hair color, seed shape, height;

Allele – controls the variation of a feature (trait).

Example: brown, blonde, black hair

REVIEW TIME: What are homologous chromosomes???

Homologous chromosomes may……-Both have the same alleles HOMOZYGOUS (aka: pure or true-breeding)-Both have different alleles HETEROZYGOUS (aka: hybrid)

Mendel’s IdeaCross two pea plants with different contrasting traits!

Ex: First cross : Crossed true breeding purple with

true-breeding white plants.

Called offspring F1 GenerationResults were that offspring were_100%

PURPLE_Had the white allele disappeared????

 

Mendel’s Law of Dominance

some alleles over power others. So even if both alleles are present, we only “see” the dominant one.

the “hidden” allele is called recessive

This only applies to SOME genes, not all

Second cross two of the purple F1 Offspring

Called offspring the F2 Generation

Results- 75 % purple- 25 % were white

White trait had reappeared!

“The Traits (genes) Mendel looked at

Mendel’s Law of Segregation

during meiosis, the pair of alleles in a parent will separate.

Only ONE allele for EACH TRAIT will pass from each parent to the offspring

Ex. sugar beet preference.dominant allele (A) prefers sugar beetsrecessive allele (a) does not. Heterozygote produces gametes

50% chanceGet AGet a

Question: If a heterozygous sugar beet eater marries a non-sugar beet eater, what possible offspring could they have?

Mendel’s Law of Independent Assortment

Alleles for different genes are passed to offspring independently of each other.

The result is that new combinations of genes present in neither parent is possible.

How many allele combinations could the following genotype produce?RRYYRRYyRrYy

Genetic TermsDiploid (2n)- Two sets of chromosomes.

Somatic Cells

Haploid (n)- One set of homologous Chromosomes(gametes)

Egg- Female haploid gamete

Sperm- male haploid gamete

Parent – Seriously, you should know this

Meiosis – Cell division that produces haploid gametes

Testes – Site of male meiosis

Gamete – Haploid sex cell (sperm, egg, pollen)

Zygote- Single cell (result of sperm and egg)

Progeny - Offspring

Offspring – see above

Fertilization – gametes fuse into zygote

Ovary- site of female meiosis - eggs

Genotype: the alleles that an organism has.

- alleles are abbreviated using the first letter of the dominant trait.

- capital letter represents the dominant - ex: P for purple flower allele

- lower case represents the recessive. - ex: p for white flower allele

All diploid organisms have two alleles for each trait:

Can be two of the same alleles Ex: PP or pp called Pure or Homozygous.

ORCan be two different alleles

Ex: Pp described as Hybrid or Heterozygous

Phenotype: physical appearance

Examples: brown hair, widows peak, purple flowers

 the trait that “wins” in the case of complete

dominance;

 depends on the combination of alleles

GENOTYPE

MENDEL’S CROSSES

P Generation: “parents;”

F1 Generation offspring of P generation

F2 Generation offspring of F1 generation

Punnet Squares

How we show allele combinations in crosses

 

 

Allele in Egg 1

Zygote formed if sperm 1 fertilizes

egg 1

Allele in Egg 2

Allele in sperm 1

Allele in sperm 2 Zygote formed if sperm 2 fertilizes

egg 1

Zygote formed if sperm 1 fertilizes

egg 2

Zygote formed if sperm 2 fertilizes

egg 2

Monohybrid CrossTall vs. Short Example

Tall allele T Short allele t

P Cross TT x tt

F1 GenerationGenotypes

Phenotypes

T T

t

t

F2 Generation

F1 Generations 100% Tt

Tt x Tt

F2 Generation

Genotypes-

Ratio =

Phenotypes-

Ratio =

t

tT

T

Sample ProblemsHomozygous Tall x Heterozygous Tall

Heterozygous Tall x Homozygous Short

ProbabilityProbability is only the LIKELIHOOD of an event

happening.

It does not mean it is what HAS to happenEx. Coin Toss. Two tosses, always one heads and

one tails?

What happens when we look at very large samples?Ex. Male/female ratio of a family vs. the world!

INHERITENCE PATTERNSEvery gene demonstrates a distinct phenotype when

both alleles are combined (the heterozygote)

Complete dominance is when both alleles are present, only the dominant trait is seen.

Incomplete dominance - when both alleles are present, the two traits blend together and create an intermediate trait

INCOMPLETE DOMINANCE

Inheritance Patterns:

Co-dominance - when both alleles are present, both traits are visible

Different notation: Use first letter of the feature with a superscript for the trait.

Example: CW or CR for white petals or red petals;

Women have two X’s but men only have one.

How do we deal with the genes on the X chromosome?

ProbabilitiesQuestion 1: What is the probability of having a

female offspring?

Question 2: After having 4 sons in a row, what is the probability the next kid will be male?

Question 3: What is the probability of having three daughters in a row?

Sex-Linked TraitsRefers to traits coded by genes found on the X

chromosomeFemales will have 2 copies of these genesMales will have 1 copy of these genes

Significance???

If males get a bad (recessive) allele for a sex-linked trait, THEY WILL EXPRESS THE RECESSIVE TRAIT!

Example – Color BlindnessSeeing color (XC) is dominant to being color blind

(Xc)

Identify the sex and trait of the following:

XCY XCXc XCXC

XcXc XcY

XC

Cross Number 1:

Xc

XC

Y

XCXC XCXc

Xc YXC Y

What % chance of having color blind daughter?

Son?

SEX-LINKED TRAITSCOLOR BLINDNESS

AFFLICTS 8% MALES AND 0.04% FEMALES.

Test cross: a cross that determines genotype of dominant parent

- Cross unknown dominant parent (possibilities BB or BB) with a recessive parent then analyze the offspring.

Ex. B- Black Hair b- white hair

You are given a black-haired guinea pig and need to determine whether homozygous dominant or heterozygous.

Multiple AllelesGenes may have more than two alleles.

Multiple alleles: Some genes have more than two variations that exist, although we still only inherit 2

Example: Human blood types

Three alleles:

IA

IB

i

Genotype PhenotypeIAIA A

IAi AIBIB B

IBi BIAIB ABii 0

Polygenic –

Multiple genes code for a trait each with 2 allelesExamples in humans:

Skin ColorEye ColorHeight

Why so many possibilities???

SKIN PIGMENTATION

Dihybrid cross:

A cross that focuses on possibilities of inheriting two traits

- two genes, 4 alleles

Black fur is dominant to brown fur

Short fur is dominant to long fur

What is the genotype of a guinea pig that is heterozygous for both black and short fur?

Dihybrid cross:

Parent phenotypes: BbSs x BbSs

Figure out the possible gametes:

Then set up punnett square

Dihybrid cross:

BS Bs bS bs

BS

Bs

bS

bs

Di-hybrid Cross Generalization

Laws of probability indicate a 9:3:3:1 phenotypic ratio of F2 offspring resulting in the following:

9/16 of the offspring are dominant for both traits3/16 of the offspring are dominant for one trait

and recessive for the other trait3/16 of the offspring are dominant and recessive

opposite of the previous proportions; and 1/16 of the offspring are recessive for both traits.

Linkage and Gene MapsWhen genes are one separate chromosomes,

they independently assort.

If on the same chromosome, they will rarely separate and be inherited together (gene linkage)

Actually it is the chromosomes that assort independently, not the genes. Mendel was just lucky with the genes he was looking at!

Crossing over in meiosis often separates linked genes.

The distance between the two genes on the same chromosome are from each other affects the frequency of separation from each other during crossing-over.Further Apart Closer together

The frequency of crossing over between genes is actually an indicator of how far apart different genes are located from each other on the same chromosome.

Use the frequency rates to make gene maps that show relative locations of genes with respect to each other.