Meiosis to the Punnett Square

How are haploid gamete cells produced from diploid cells?

Your parents have 46 chromosomes each and you have 46 chromosomes.

If you got half of your genes from each parent, how did this happen? How many chromosomes do a sperm and

an egg have?

Meiosis

The process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell that become haploid cells.

Involves two distinct divisions; Meiosis I and Meiosis II

One diploid cell becomes 4 haploid cells

Crossing Over

Prophase I Homologous

chromosomes exchange alleles

Produces new combinations of alleles

Causes randomness in gamete formation

Meiosis I

Prophase I- Chromosomes pair with same homologous chromosomes to form a tetrad.

Metaphase I- Homologous chromosomes meet in the middle and spindle fibers attach.

Anaphase I- Homologous chromosomes are pulled apart by spindle fibers

Telophase I- Nuclear membrane forms around separated chromosomes to form two nuclei.

Cytokinesis- The diploid cell separates into two haploid cells.

Meiosis II starts with two haploid daughter cells Prophase II- haploid chromosomes with sister

chromatid are in the cytoplasm. Metaphase II- chromosomes meet in the

middle of the cell Anaphase II- Sister chromatid are pulled apart

by spindle fibers Telophase II- Nuclear envelop forms around

the chromosomes Cytokinesis- Results in 4 haploid daughter

cells

Meiosis animation:

Pay close attention to what “meets” in each metaphase and what separates in each of the two anaphases.

http://www.cellsalive.com/meiosis.htm

Review

Mitosis Both Meiosis

Place these items into your Venn Diagram:

Produces diploid cells Produces haploid cells Type of cell division Creates identical cells Separates homologous

chromosomes Separates chromatids Creates “daughter” cells

2 rounds of cell division Only 1 round of cell

division Creates unique cells Begins with diploid cells Creates gametes Creates “body cells” Has crossing over

Genes

Genes are the characteristics contained in the DNA of every cell.

Each gene is made up of 2 alleles- one inherited from each parent.For instance you may have one brown eye

allele inherited from your dad and one blue eye allele inherited from your mom.

Kinds of alleles

Dominant- These alleles always determine the physical feature

of the individual. These alleles are represented by a capital letter.

Recessive- These alleles only determine the physical feature of

the individual when there is no dominant allele. These alleles are represented by a lower case

letter.

Genotype and Phenotype

The genotype of an individual is the description of the actual genetic information for a trait.

The phenotype of an individual is the physical feature observed.

Allele Combinations

Every gene is made up of two alleles-one from each parent.

If both alleles are the same, the gene is called homozygous. (BB or bb)

If the alleles are different, the gene is called heterozygous. (Bb)

Label the following as homozygous dominant, homozygous recessive, or heterozygous.

1. BB

2. Aa

3. nn

4. Kk

5. GG

6. ff

7. qq

8. Hh

Probability and Punnett Squares

Meiosis separates the alleles of a parent into different sex cells (gametes).

For instance: Dad is heterozygous for Brown hair, with a blond hair allele as the recessive. (Bb).Through meiosis, these alleles are copied

and separated into 4 sperm.

B B b b

Single Trait Crossing

What kind of offspring (kid) would two parents have?

A Punnett Square can help you find out!Example: Dad is heterozygous (Bb) for

Brown hair over blond hair. Mom is also heterozygous (Bb) for brown hair over blond hair.

Set up the Square

Dad’s Alleles

Mom’s alleles

B

B

b

b

Set up the Square

Dad’s Alleles

Mom’s alleles

B

B

b

b

BB Bb

Bb bb

4 possible offspring

What do these results mean?

¼ or 25% chance of an offspring with homozygous dominant genes for brown hair.

½ or 50% chance of an offspring with heterozygous genes for brown hair.

¼ or 25% chance of an offspring with homozygous recessive genes for blond hair.

You try it!

1. Dad is heterozygous for Brown eyes over blue eyes. (Ee) Mom is homozygous recessive with blue eyes. (ee) What are the probabilities for the children’s genotypes. What are the probabilities for the children’s phenotypes?

Set up the Square

Dad’s Alleles

Mom’s alleles

E

e

e

e

Dad’s Alleles

Mom’s alleles

E

e

e

e

Ee ee

Ee ee

4 possible offspring

½ or 50% of the children would inherit heterozygous genes for Brown eyes (Ee)

½ or 50% of the children would inherit homozygous recessive genes for Blue eyes (ee)

Hmmm…

Is it possible for two Brown haired parents to produce a blond child? Explain.

Is it possible for 2 blue eyed parents to produce a brown eyed child? Explain.

Can you figure out your genotype for hair color? How do you know?

Review Questions

1. How is meiosis related to heredity?

2. What is the difference between heterozygous, homozygous dominant, and homozygous recessive?

3. Is it possible for two brown haired parents to produce a blond child? Explain.

Review cont…

4. What is the difference between genotype and phenotype?

5. What are the chances that two heterozygous parents with Brown hair(Bb) produce a child that is also heterozygous?

Review cont…

6. In pea plants, round peas are dominant over wrinkled peas. (R and r). What would the offspring of a homozygous dominant, and a homozygous recessive cross look like?

7. If the offspring from question #6 were to pollinate each other (as plants often do), what would be the genotype probabilities for the offspring?

Two Trait Crosses

When looking at two different genes in the same cross, 4 alleles are dealt with.For instance, a heterozygous haired(Bb),

and homozygous recessive eyed(ee) parent would be Bbee.

Through meiosis, there would be 4 possible allele combinations in the sex cells.

FOIL!

Just like in math class, the first, outside, inside, and last rule can help you remember.

B b e e These alleles would produce these four

combinations.Be Be be

be

What does a two trait cross look like?

Suppose two parents who are both heterozygous haired and heterozygous eyed produce a child. What are the possible genotypes and phenotypes in the offspring?

You try it!

A tall (TT) pea plant that produces yellow colored peas (Yy) crosses with a short (tt) pea plant with green colored peas (yy). Create the Punnett square and list the probabilities of each genotype.