Theoretical GeneticsTheoretical Genetics

Gregor MendelGregor Mendel

ObjectivesObjectives

4.3.1 – Define dominant allele, recessive allele, codominant alleles, locus, homozygous, heterozygous, genotype, phenotype,

carrier, and test cross.

4.3.2 – Determine the genotypes and phenotypes of the offspring of a monohybrid cross using a Punnett grid.

Review of DNA StructureReview of DNA StructureEukaryotic DNA is divided among

several chromosomes (humans have 23 pairs of

chromosomes – one set from each parent).

Chromosomes are divided into units called genes.

Genes code for the body’s proteins: eye pigment.Genes come in versions called alleles

Eye color gene – either brown

or blue allele.

Result of mutation.

DefinitionsDefinitionsLocus

The particular position of a gene on homologous chromosomes.

Homologous: codes for the same things.

DefinitionsDefinitionsHomozygous vs. heterozygous alleles

An organism with two identical alleles for a character is homozygous for that character.Organisms with 2

different alleles for a character

are heterozygous.

Bb BB bb

DefinitionsDefinitionsDominant & recessive allele (complete

dominance)A dominant allele is strong and is expressed (or

seen) in the phenotype. (Abbreviated with capitals)

A recessive allele (if present in a heterozygous individual) is not

expressed; it is hidden because it is

weak. Reces- sive alleles only show up if the in-dividual is homo-zygous. (Lowercase

letters)

Bb BB bb

DefinitionsDefinitionsIncompletely dominant alleles

Sometimes, neither allele (when heterozygous) can overpower the other. They produce a blend.

= allele for white flower color (Incomplete dominance)= allele for red flower color (Incomplete dominance)

Pink Red Whiteflowers flowers flowers

DefinitionsDefinitionsCodominant alleles

Pairs of alleles that both affect the

phenotype when present in a heterozygote.

Ex: Blood groups:A, B, O are allelesalleles that code for a sig-

nal protein on the membrane surface. O is recessive, but

A & B codominant.

Punnet squarePunnet square

DefinitionsDefinitionsGenotype vs. phenotype:

All the many alleles of an organism (for eye color, hair color, seed appearance, etc.) make up its genotype [genetic make-up; think: type of genes].An organism’s physical characteristics make a phenotype.

The science of geneticsThe science of geneticsGenetics - the study of heredity.

Heredity – a characteristic of life: the passing of traits from parents to offspring.

Traits - characteristics such as eye color, body size, sickle cell anemia, etc.

The science of genetics be- gan with an Austrian

monk named Gregor Mendel in the 1860s, working with pea plants (Pisum sativum).

The science of geneticsThe science of geneticsMendel studied pea flower, seed & pod

color, and seed shape.Did careful pollination work & counted offspring.

Ex: he mated plants with white flowers to plants with purple flowers, or plants with wrinkled seeds to plants with smooth seeds (the parental generation, or P).

The science of geneticsThe science of geneticsMendel’s work with peas:

Mendel found that plants of the first generation (F1) were often identical.

Ex: all had purple flowers or all had yellow seeds. →→

But mating the first generation (F1) plants gave a 3:1 ratio in the

2nd generation (F2).

*F stands for filial (son)

F1 generation

F1 generation

Mendel’s Law of SegregationMendel’s Law of SegregationA pair of allelic genes for a particular

character, like eye color, separate (segregate) in equal ratios during gamete formation.

During meiosis, alleles are separated on a 1:1 ratio into sperm and eggs

Ex: if the genotype = Gg, half the sperm must contain G and the other half must contain g.

Punnet gridsPunnet gridsTo make a Punnet grid:

1) Determine the geno- type of the parents.

2) Make every possible combination of gamete.

3) Combine the sperm and the egg.

4) Determine the phenotypes from

the genotypes of the offspring.

P represents the gene that makes thepurple flower pigment. The allele p ismutated, and the protein is defective.All F1 offspring are Pp and purple.

Male parent

Female parent

A monohybrid cross:one locus is considered

Sperm

Eggs

Punnet gridsPunnet gridsTo make a Punnet grid:

1) Determine the geno- type of the parents.

2) Make every possible combination of gamete.

3) Combine the sperm and the egg.

4) Determine the phenotypes from

the genotypes of the offspring.

F1

generation

F2

generation

Punnet gridsPunnet gridsIncomplete dominance

Ex: red and white snapdragon flowers

Blending of color

A 1:2:1 ratio of offspring

Test crossTest crossTo test an unknown individual:

Testing a suspected heterozygote by crossing it with a

known homozygous recessive individual.

1 of 2 possible outcomes shows

the genotype of the

unknown parent.

ObjectivesObjectives

4.3.3 – State that some genes have more than two alleles (multiple alleles).

4.3.4 – Describe ABO blood groups as an example of codominance and multiple alleles.

Mendel’s Law of Independent AssortmentMendel’s Law of Independent AssortmentAlleles of genes on nonhomologous

chromosomes assort independently during meiosis.

All blonds do not have blue eyes.Hair color & eye color are on different chromosomes.

Chromosomesare shuffled.

Punnet gridsPunnet gridsTo make a dihybrid cross(crossing 2 pairs of genes):

1) From parents’ genotypes, determine gametes.

Make every possiblecombination of gametes.(independent assortment)

every possible combination

Punnet gridsPunnet gridsTo make a dihybrid cross:

1) From parents’ genotypes, determine gametes.2) Combine the sperm and eggs, and determine

phenotypes.

Multiple allelesMultiple allelesCodominant alleles

Pairs of alleles that both affect the

phenotype when present in a heterozygote.

Ex: Blood groups:A, B, O are allelesalleles that code for a sig-

nal protein on the membrane surface. O is recessive, but

A & B codominant.

Punnet squarePunnet square

Multiple allelesMultiple allelesSome genes have more than two alleles.

ABO blood group shows codominance of multiple alleles (IA and IB are codominant; i is recessive).

O (ii) is the universal donor; AB accepts any blood.

Punnet squarePunnet squarePunnet square for blood groups

I is dominant, but there are 2 types: IA & IB

i is recessive.

Here, a mother with blood group O mates with a father who’s AB.

Punnet squarePunnet squarePunnet square for sex-linked traits

For genes found on a sex chromosome, X or Y.

Remember, boys have only one X chromosome, so they are more likely to get these diseases, like color-blindness or hemophilia.

Notice, 50% of babies are boys, and 50% are girls. Girls get 2 X’s, sothey are likely to have a good back-up copy and don’t get affected.

Multiple allelesMultiple allelesPolygenic inheritance

Two alleles on each of three genes have

an additive effect on skin color.

Genetic diseasesGenetic diseasesSickle-cell anemia

The gene for hemoglobin is mutated.An individual with two copies of the recessive allele can’t move oxygen around the body well.

Genetic diseasesGenetic diseasesSickle-cell anemia

A carrier is an individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this allele.

Carrier genotype = Hhs - resistant to malaria

Both Hhs

HH hshsHhs

The gene for hemo- globin (H) is mutated.

Homozygous recessiveindividuals often dieyoung.

Sex-linkage and Pedigrees

Sex-linkage and Pedigrees

Diseases linked to sex chromosomesDiseases linked to sex chromosomesSex-linked traits: genes are on the sex

chromosomesXX individuals are female XY individuals are male.

Theoretically 50:50 male:female.Actually ~51:49 male:female at birth then 50:50 by age 3 (boys weaker).

Diseases linked to sex chromosomesDiseases linked to sex chromosomesColor-blindnessColor-blindness is caused by a defective gene for a pig- ment receptor in the eye. Gene is on X chromosome.

Boys only get 1 X, so they are more likely than girls to get this problem.

Diseases linked to sex chromosomesDiseases linked to sex chromosomesColor-blindnessColor-blindness is caused by a defective gene for a pig- ment receptor in the eye. Gene is on X chromosome.

Boys only get 1 X, so they are more likely than girls to get this problem.

Retina of the eye

Diseases linked to sex chromosomesDiseases linked to sex chromosomesHemophiliaHemophilia: a genetic disease in which the

body does not produce sufficient amounts of a clotting factor. As a result, fibrin (necessary to maintain the blood clot) does not form, so the individual is more likely to bleed long- er from a wound, and to

bleed internally.The gene for this clotting

factor is on the X chromo-some.

Boys have noback-up copyof the X.

Genetic diseases - sex linkageGenetic diseases - sex linkageHemophiliaHemophilia: genotypic & phenotypic ratios

*Note: the daughters can be either heterozygous for sex-linked diseases or homozygous if a carrier mother marries a diseased man. 1 in 10,000 males is affected with hemophilia, 1 in 10,000 males is affected with hemophilia, and 1 in 100,000,000 females.and 1 in 100,000,000 females.

The female carrier is heterozygous*The female carrier is heterozygous*

Genetic diseases Genetic diseases HemophiliaHemophilia: A case study

Alexei, tsarevich of Russia inherited the disease from his mother.

The original mutation in this lineage occurred in Queen Victoria.The original mutation in this lineage occurred in Queen Victoria.

PedigreesPedigreesX-linked dominant trait passed from the father:

Only his daughters get the disease.

PedigreesPedigreesY-linked trait (carried on the Y chromosome).

Only boys get the disease.

KaryotypeKaryotypeA karyotype is a picture of the body’s

chromosomesIt shows abnormalities, also the individual’s sex.

KaryotypeKaryotypeA karyotype is a picture of the body’s

chromosomesIt shows abnormalities, also the individual’s sex.

Down Syndrome – three copies of chromosome 21.