Genetics

GeneticsCh. 11 pgs. 263-274

What is Genetics?

• The study of heredity

• Passing of traits from one generation to the next.

Gregor Mendel (1822)

• Monk who worked in garden of monastery in Czech Republic.

• Used pea plants to study

heredity

• Known as the “father

of modern genetics”

Why Pea Plants?

• Many contrasting traits

• Reproduce sexually

• Crosses can be controlled

• Large # of offspring

• Short lifecycles

A little more about pea plants…

• Normally self pollinating

• Pollen (sperm) fertilizes the ovule (egg) to produce offspring.– Produce genetically identical offspring.

• Mendel used cross-pollination in his experiments.• Used pollen from one plant to fertilize another.

Genes and Dominance

• Mendel studied 7 traits of pea plants– Traits are specific characteristics – For each trait there are 2 contrasting

characteristics.

Generations

• P- parental generation; original pair

• F1- first filial generation; offspring from P generation

• F2- second filial generation; offspring from F1

Mendel’s Findings

• When he crossed P generation, F1 showed only one trait from the 2 parents.– Ex. Crossed Tall x Short short or tall

Mendel’s Conclusions

• Biological inheritance is determined by factors passed from one generation to the next.– Factors are called genes.– 2 contrasting forms of the same trait are

called alleles.• Ex. Trait is height, alleles are tall and short

Mendel’s Conclusions

• Principal of dominance- Some alleles are dominant and some are recessive.– Dominant- when allele is present organism

will always exhibit the dominant trait.– Recessive- only exhibit recessive trait if

dominant allele is not present.

Alleles

• Symbols for alleles:– A letter symbol is used for a specific trait.

• Ex- height, Tt

• Dominant- symbol is uppercase letter, T

• Recessive- symbol is lowercase letter, t

Segregation

• When Mendel crossed plants from the F1 generation traits from the P generation reappeared.

P Tall x Short all tall plants

F1 Tall x Tall some tall some short

Law of Segregation

• The 2 alleles for each trait must separate when gametes form.

– Parent only passes one

allele for each trait to its

offspring.

Probability and Punnett Squares

• The principles of probability can be used to predict outcomes of genetic crosses.

• Punnett square- diagram

which shows possible

Results of genetic crosses

Heterozygous vs. Homozgous

• Hetero different

• Homo same

• Homozygous dominant- 2 identical dominant alleles (upper case). Expresses the dominant trait.– Ex- TT, tall

Heterozygous vs. Homozygous

• Homozygous recessive- 2 identical recessive alleles (lower case). Expresses the recessive trait.– Ex- tt, short

• Heterozygous- Have 2 different alleles for the same trait. ( upper and lower case) Express the dominant trait.– Ex. Tt, tall

Genotype vs Phenotype

• Genotype- genetic make-up, letters– Tt vs tt

• Phenotype- physical

expression of the trait– Tall vs short

Sample Problem

• In summer squash, white fruit color (W) is dominant over yellow fruit color (w). If a squash plant homozygous for white is crossed with a plant homozygous for yellow what will the phenotypic and genotypic ratios be?

Sample Problem

• If 2 heterozygous white plants are crossed what will be the phenotypic and genotypic ratios?

Types of Genetic Crosses

• Monohybrid one trait– Ex. Tt x tt

• Dihybrid 2 traits– Ex- Hair and height– B= black, b=blonde, H=tall, h=short– What would be the phenotypes of a cross

between the following parents?

HhBb x HHbb

Types of genetic crosses cont.

• Ex 2- Shape and color of pea plants

R= round, r=wrinkled, Y= yellow, y=white

What would be the phenotypes of a cross between a plant that is RRYY and a plant that is rrYy?

Genetics Review

• If normal vision (N) is dominant to colorblindness (n) what are the chances that a heterozygous normal man and a colorblind woman will have a child with colorblindness?

Genetics Review

• In some flowers red flowers (R) are dominant to white flowers (r) and tall stems (T) are dominant to short stems (t). What is the genotypic ratio for a cross between the following plants:

RrTT x rrTT

DD:DR:RD:RR

Types of genetic crosses cont.

• Incomplete dominance– One allele is not completely dominant over

the other– Heterozyous phenotype is somewhere

between the 2 homozygous phenotypes.• 3 phenotypes instead of 2

– Use all capitol letters• Ex; RR= red, WW= white, RW= pink

Types of genetic crosses cont.

• Inc. Dom. Example:In snapdragons the combination of red and white flowered plants can produce a pink flowered plant.

RR= red WW= white RW= pink

What would be the phenotypes of the offspring if a red flower were crossed with a pink flower?

Types of genetic crosses cont.

• Co-dominance– Both alleles contribute to the phenotype– Ex- BB= black feather, WW= white feather,

BW= black and white spotted feathers.– Same rules as inc. dom. but both phenotypes

appear separately

Types of genetic crosses cont.

• Co-dominance sample problem

We have 2 fuzzy bunnies in class. One has black and white fur, the other is pure white. What are the genotypes of both rabbits?

Black and White= White=

What would be the phenotypes of the offspring if these rabbits mated?

Blood Type

• Human blood type is determined by the type of protein found on the red blood cells (A or B).

• Antigen- protein located

on blood cell

• Antibody- protein found in

plasma, prevent foreign

particles

Blood type

Blood type

Possible genotype

Antigen on RBC Antibodies in Blood

Type that can be received

A iAiA

iAi

A B A, O

B iBiB

iBi

B A B, O

AB iAiB A & B None AB, A, B,

O

O ii None A & B O

Blood Type Example

• What are the chances of a mother with A type blood and a father with O type blood having a child with A type blood?

Polygenic Inheritance

• Some traits are polygenic- have more than one gene coding for the trait.– Ex- skin color, eye color, height

Polygenic Inheritance cont

• 1. The weight of a fruit in a certain variety of squash is determined by two pairs of genes: AABB produces fruits weighing 4 lbs each and aabb produces fruits weighing 2 lbs each.

A. How many pounds does each dominant allele add to the total weight of an individual squash?

Human Genetics

• Chromosomes and Sex Determination– Humans have 23 pairs of chromosomes in

autosomal cells. (diploid)

• In sex cells there are only 23. (haploid)

• Sex chromsomomes are the only pair that are non-homologous.– XX female– XY Male

Human Genetics

• Ex- Cross between male and female for sex chromosomes:

– Chance of boy:– Chance of girl:

Sex-Linked Inheritance

• Some traits are linked to the X chromosome.

• Most genetic disorders are X-linked.

• Most X-linked disorders are expressed in males.

WHY???

Sex- Linked Inheritance

• Females usually act as carriers, they carry the recessive trait on one of their X chromosomes but the trait is not expressed.

Sex-Linked Inheritance

• Ex- Normal colored vision (N) is dominant to colorblindness (n). Colorblindness is an X-linked trait. – Males are colorblind more then women and

the gene is only found on the X chromosome.

What is the chance that a man with normal vision and a woman who is a carrier will have a child that is colorblind?

Autosomal Traits

• Genetic traits that are carried on the autosomal chromosomes.

Autosomal Recessive

• Only expressed with homozygous recessive genotype.

• Heterozygous genotype

is carrier.

• If trait present usually

lethal disorder.– Ex- cystic fibrosis

Autosomal Recessive

• Ex-

C= no cystic fibrosis c= cystic fibrosis

A female homozygous dominant for cystic fibrosis marries a man heterozygous for cystic fibrosis. What is the probability the child will have cystic fibrosis?

Autosomal Dominant

• Expressed in homozygous dominant and heterozygous genotype.

• No carrier (Either you have it or you don’t)

• If trait is present usually

lethal.– Ex- Huntington’s Disease

Autosomal Dominant

• Ex

H= Huntington’s disease h= normal

A female who has Huntington’s disease and is heterozygous for the disorder marries a man who does not have the disorder. What is the probability that their child will have the disorder?

Pedigrees

• Chart which shows the relationships within a family.– Shows the expression of genetic traits.

• Symbols used:– Circle=female– Square=male– Half-shaded=carrier– Completely shaded=expresses trait

Pedigree

Pedigree

• Used to determine if disorder is autosomal dominant, autosomal recessive or X-linked.– If only expressed in males and carried in

female X-linked– If there are many carriers of both sexes

autosomal recessive.– If no carriers and expressed in both sexes

autosomal dominant

Pedigree examples

Pedigree Examples

Pedigree Examples