Introduction to genetics

Ch. 11

Meiosis….Some terms to know• Haploid (1n)- a cell

with only one complete set of chromosomes (gametes or sex cells). Diploid (2n)- a cell that contains two complete sets of chromosomes. (all other cells)

How do you know sex cells (eggs and sperm) are only haploid?Which are diploid and which are haploid?

Homologous Chromosomes come from a Tetrad

• Homologous chromosomes are pairs of chromosomes where each one is identical to the other.

• A tetrad is a structure containing 4 strands of DNA tightly paired together.

Meiosis

• There is a meiosis I and II. Each are the same steps as mitosis.

• You begin with one diploid cell and produce 4 haploid.

• This gives you many different combinations of genes to be passed on. It’s all chance on the ones you get.

Crossing Over

• Occurs during prophase 1 when homologous chromosomes pair up and form tetrads and exchange portions of chromatids.

Meiosis Provides for Genetic Recombination

• 1.28a Sorting and recombination of genes in sexual reproduction, allows for a great variety of possible gene combinations.

• That is why no two people are alike. (Besides identical twins- why?)

1.46 A great diversity of species increases the chance that at least some living things will survive in the

face of large changes in the environment.

• Why is this?• So, is diversity good?

Give an example.

The Steps…Let’s review.

• Prophase I• Metaphase I• Anaphase I• Telophase I• Prophase II• Metaphase II• Anaphase II• TelophaseII

• What occurs during each step?

1.8c Mitosis vs. Meiosis

• Mitosis

– Somatic cells

– One cell division

– Two daughter cells with same number of chromosomes as parent.

– Daughter cells are identical to parent.

– Asexual

• Meiosis

– Sex Cells

– Two cell divisions

– Four daughter cells with half chromosomes as parent

– Daughter cells not identical to parents.

– Sexual

Genetics

• Genetics is the study of heredity.

• What is heredity?• Genetics involve traits

being passed from one parent to offspring.

• What are some traits you got from your parents?

Fertilization

• Gametes- Sex Cells– Sperm and Egg

Fertilization- process of sperm fertilizing the egg.

Zygote- fertilized egg

Embryo- zygote- 8 weeks.

Fetus- 9 weeks - delivery

Gregor Mendel..The Father of Genetics

• Mendel studied peas for 3 reasons:

1. Structure of pea

2. Presence of distinctive traits.

3. Rapid reproduction cycle. 90 days

• What are the male and female parts of a flower?

• How do you think he cross fertilized them?

Purebred vs. Hybrids

• Since flowers have both male and female (anther in stamen and ovary in pistol), they are able to self-fertilize.

• True-breeding- if org. were allowed to self fertilize, they would have offspring identical to self.

• (pure-bred, same genetic traits from each parent)

• A hybrid is an organism that receives different forms of a genetic trait from each parent.

Generations

• P is the Parental Generation.

• F1- what you get when the parents reproduce. YOU.

• F2- The offspring of F1. YOUR KIDS.

What are P, F1 and F2 in these pictures, assuming everyone is from the same family?

Mendel’s Experiments

• He tested 7 traits of the pea pod:

1. Flower Color

2. Flower position

3. Pea color

4. Pea shape

5. Pod color

6. Pod shape

7. HeightMendel’s Peas

Mendel’s Study• Cross pollinated pea plants,

taking a stamen and putting that into the pisil of another flower using two pure-bred plants. All of them had yellow peas.

• Allowed them to self-fertilize. He got 75% yellow and 25% green.

• He thought they would blend to form a chartreuse color? Why?

• He ended up crossing all 7 traits. See Figure 11-3.

Mendel’s Results

P- Green X yellow

F1- Yellow

F2- Green and Yellow

Principles of Inheritance• Chromosome Theory

of Heredity states that genes in chromosomes carry the material of inheritance.

1.21 Genes are coded in DNA molecules and pass information from parents to offspring.

• Genes are sections of a chromosome that code for a traits.

• Alleles are one form of a gene. – Ie. Eye color can be

blue, green, brown, etc.– You receive one allele

from your mom and one from your dad.

• Alleles can be dominate or recessive.

• Dominate- always expressed.

• Recessive- only expressed if no dominate is present.

• Let me explain.• BB, Bb, bb

Allele Expression

• Homozygous dominant- HH (pure)

• Heterozygous dominant- Hh (hybrid)

• Homozygous recessive- hh (pure)

• If H is long hair and h is short hair, which will have long and which short? (on a cat)

Genotype and Phenotype

• Genotype represents the genetic make up.

HH

Hh hh

• Phenotype represents the outward expression.

Long Hair

Long Hair

Short Hair

Review of Terms

Mendel’s laws

• 1.22a Law of Segregation states that allele pairs segregate during meiosis.

• What does segregate mean?

• 1.22b Law of Independent Assortment states that genes for different traits can segregate into gametes randomly and independently of each other. Chromosomes not genes assort independently.

• Law of Dominance states that the dominant allele is expressed and the recessive allele can be hidden.

A a A a

Mom Dad

Genetics and Predictions

• Punnett Squares are used to determine probabilities of outcomes for offspring.

• Let’s make one.• What do you get in each

box?• What are the genotypes and

phenotypes?• What are their probabilities?

A a

A

a

Monohybrid and Dihybrid Crosses

• Monohybrid crosses deal with one trait.

• Dihybrid crosses deal with 2 traits.

• Let’s do one of each.

• If R is dominant and is round, and Y is dominant and is yellow and you cross RrYy x RrYy, what do you get?

Test CrossesTest Crosses are used to

distinguish between homozygous dominant and heterozygous organisms.

You breed an organisms unknown genotype with a homozygous recessive organism.What are the outcomes?

Show how this is helpful and works!

y

y

y

y

Y Y

Y y

Y=Yellow, y=green

Difficult Predictions

• Incomplete Dominance is when two different alleles for the same trait combine.

R=red, W=white

RW= Pink

• Codominance is when both alleles express themselves fully.

Blood type.

A, B or AB

Polygenic Traits

• Polygenic traits are controlled by two or more genes.

• Eye color- many genes control the pigment (tone, amount, and position).

• Skin tone, hair color, height

Multiple alleles and Pleitropy• Multiple alleles are

when three or more alleles are found in the population.

• Ie. Rabbits have 4 different alleles for hair color, but each rabbit only has 2 of them.

• Blood types

• Pleitropy occurs when a single gene affects more than one trait.

• Sickle Cell anemia.

Environmental Effects

• 1.1b Different parts of the genetic instructions are used in the different kinds of cells and are influenced by the cell’s environment and past history.

• Himalayan rabbits fur color is affected by temperature.

• Western white butterfly wing coloration is affected by temp.

• Japanese goby fish can change its sex back and forth in response to changes in its social environment.

.

Linked Genes

• Genes that are located on the same chromosome, which are inherited together.– Ie. Fruit fly’s red eyes and

miniature wings inherited together.

– Those closer together, more likely to be inherited together.

• It is the chromosomes that assort independently, not the genes.

Genetic Mapping

• Researchers have made gene maps that show what traits(alleles) are on what chromosomes and where.

• The farther apart the 2 genes are located, the more likely they are to be separated by a crossover. (Cross over more frequently)

Genetics Review

• Go to the following link and click on your book. Take the self-test and review the Active Art.

• Ch. 11 Review