Mendel and Meiosis

Unit 4

Chapter 10

Gregor Mendel

Austrian monk

Studied patterns of heredity (passing on of characteristics from parent to offspring)

Used the common garden pea in experiments

Why did Mendel use peas?

Sexually reproducing: able to isolate both male and female gametes

Easy to identify traits (characteristics that are inherited)

Short life cycle: able to be grown quickly

Hybrid

Any offspring of parents with different traits (ex: tall plant x short plant)

Monohybrid cross: cross-pollination (breeding) between two parents with only one variation difference (ex: tall plant x short plant)

Dihybrid cross: cross-pollination (breeding) between two parents with two variation differences (ex: tall, green plant x short, yellow plant)

Pea cross-pollination experiments

PARENT GENERATION (P1)

Tall true breed x short true breed

FILIAL GENERATION (F1)

All tall hybrids

FILIAL GENERATION (F1)

75% tall hybrids, 25% short hybrids

Phenotypes from P1 to F2Dihybrid Cross round yellow x wrinkled green

Round yellow Wrinkled green

All round yellow

Round yellow Round green Wrinkled yellow Wrinkled green

9 3 3 1

P1

F1

F2

Mendel’s conclusions

There must be two variations for every trait, where each variation is called an allele.

Each offspring inherits only one allele from each parent.

The alleles are either dominant or recessive.

To show the recessive trait, two recessive alleles must be inherited.

Dominant and recessive traits

Recessive trait

Dominant trait

Seed shape

Seed color

Flower color

Flower position

Pod color

Pod shape

Plant height

round yellow purple

axial (side) green inflated tall

wrinkled green whiteterminal

(tips) yellow constricted short

Homozygous vs. Heterozygous

Homozygous: inherits two similar alleles from the parents for a particular gene Ex: tall allele and tall allele, written as TT Ex: short allele and short allele written as tt

Heterozygous: inherits two different alleles from the parents for a particular gene Ex: tall allele and short allele, written as Tt

Using a Punnett square

AA x aa 100% Aa Each of the four squares represents 25%

chance of inheritance for one offspring.

A Aa A a A a

a A a A a

Punnett square video

Click on image to start video.

Phenotype vs. Genotype

Phenotype: physical appearance of the trait Ex: purple flowers

Genotype: homozygous or heterozygous inheritance Ex: PP, Pp, pp

A Punnett square for a dihybrid cross will need to be four boxes on each side for a total of 16 boxes.

Dihybrid crossesDihybrid crossesPunnett Square of Dihybrid Cross

Gametes from RrYy parent

RY Ry rY ry

Ga

me

tes

fro

m R

rYy

pa

ren

t

RY

Ry

rY

ry

RRYY RRYy RrYY RrYy

RRYy RRYy RrYy Rryy

RrYY RrYy rrYY rrYy

RrYy Rryy rrYy rryy

F1 cross: RrYy ´ RrYy

round yellow

round green

wrinkledyellow

wrinkledgreen

Punnett Square of Dihybrid Cross

Gametes from RrYy parent

RY Ry rY ry

Ga

me

tes

fro

m R

rYy

pa

ren

t

RY

Ry

rY

ry

RRYY RRYy RrYY RrYy

RRYy RRYy RrYy Rryy

RrYY RrYy rrYY rrYy

RrYy Rryy rrYy rryy

Dihybrid crossesDihybrid crosses

Cells and chromosomes

A cell with two of each kind of chromosome is called a diploid cell and has diploid, or 2n, number of chromosomes.

Organisms produce gametes that contain one of each kind of chromosome

Homologous chromosomes

• The two chromosomes of each pair in a diploid cell are called homologous chromosomes.

Homologous chromosomes

• On homologous chromosomes, the same types of genes are arranged in the same order.

• Because there are different possible alleles for the same gene, the two chromosomes in a homologous pair are not always identical to each other.

Making haploid cells

Meiosis is the process of producing haploid gametes with a ½ the amount of DNA as the parent cell.

A cell with one of each kind of chromosome is called a haploid cell and has a haploid, or n, number of chromosomes.

Meiosis enables sexual reproduction to occur.

Sexual reproduction

Meiosis

Meiosis

Sperm Cell

Egg Cell

Haploid gametes

(n=23)

Fertilization

Diploid zygote

(2n=46) Mitosis and Development

Multicellular

diploid adults

(2n=46)

Meiosis process

Click on image to play video.

Interphase

• During interphase, the cell replicates its chromosomes.

• After replication, each chromosome consists of two identical sister chromatids, held together by a centromere.

Prophase I

The chromosomes coil up and a spindle forms.

Homologous chromosomes line up with each other gene by gene along their length, to form a four-part structure called a tetrad.

Prophase I – crossing over

Chromatids are wrapped so tightly the chromosomes can actually break and exchange genetic material in a process known as crossing over.

Crossing over results in new combinations of alleles on a chromosome.

Metaphase I

• The centromere of each chromosome attaches to a spindle fiber.

• The spindle fibers pull the tetrads into the middle, or equator, of the spindle.

Anaphase I

• Homologous chromosomes separate and move to opposite ends of the cell.

• This critical step ensures that each new cell will receive only one chromosome from each homologous pair.

Telophase I

• The spindle is broken down, the chromosomes uncoil, and the cytoplasm divides to yield two new cells.

• Each cell has half the DNA as the original cell because it has only one chromosome from each homologous pair.

Prophase II

A spindle forms in each of the two new cells and the spindle fibers attach to the chromosomes.

Metaphase II.

The chromosomes, still made up of sister chromatids, are pulled to the center of the cell and line up randomly at the equator.

Anaphase II

The centromere of each chromosome splits.

The sister chromatids to separate and move to opposite poles.

Telophase II

• Finally nuclei reform, the spindles breakdown, and the cytoplasm divides.

• Four haploid cells have been formed from one diploid cell

Why meiosis is important

1. Forms gametes for sexual reproduction

2. Crossing over during meiosis which rearranges allele combinations so that the offspring generations are genetically different than the parents.

Nondisjunction leading to trisomy

This can lead to gamete formations having too many or too few chromosomes.

Ex: A gamete with 2 copies of #21 chromosome fertilizes a gamete with 1 copy of #21. The result is an embryo with trisomy 21. This causes Down Syndrome in humans.

Trisomy leading to monosomy

A gamete with one copy of the X chromosome fertilizes a gamete missing a copy of the X chromosome.

The result is monosomy X, which in humans causes Turner Syndrome.

Nondisjunction leading to polyploidy

• When a gamete with an extra set of chromosomes is fertilized by a normal haploid gamete, the offspring has three sets of chromosomes and is triploid.

• The fusion of two gametes, each with an extra set of chromosomes, produces offspring with four sets of chromosomes and is a tetraploid.

• This occurs often in flowering plants, leading to larger fruit production.