Chapter 10 Mendel and Meiosis

Section 10.1Mendel’s Laws

of Heredity

Why Mendel Succeeded:• Mendel chose his subjects

carefully-He chose the garden pea plant because they

reproduce with sex cells called gametes.

• Mendel was a careful researcher-He studied only one trait at a time to control

variables, and he analyzed his data mathematically.

Mendel’s Monohybrid Crosses

The First and Second Generations

Mendel he cross pollinated short and tall plants and in the first generation it appeared as if the short gene of the plant never existed.

However, in the second generation, he let them self pollinate, and he found that three-forth’s of the generation were tall and the other forth was short.

The Rule of Unit Factors and the Rule of Dominance

The rule of unit factors is a conclusion that each organism has two factors that control each of it’s traits. These factors are genes that are located on chromosomes and the genes exist in alternative forms called alleles.

When Mendel crossed the tall plant with the short plant, the first generation plants were small which means only one trait was observed. He called the observed trait dominant and the trait that disappeared recessive.

The Law of SegregationWhen the first generation then reproduced the result was a trait of shortness. The law of segregation basically states that because each plant has two different alleles, it can produce two different types of gametes. During fertilization, male and female gametes randomly pair to produce four combinations of alleles.

Phenotypes and Genotypes

Two organisms can look alike but have different underlying allele combinations. The way and organism looks and behaves is called its phenotype. The phenotype of a tall plant is tall, whether it is TT or Tt.

The allele combination an organism contains is known as its genotype. The genotype of a plant that has two tall genes is TT.

An organism is homozygous for a trait if its two alleles for the trait are the same. The breeding tall plant that had two alleles for tallness would be homozygous for the trait of height. Because tallness is dominant for that trait, a TT individual is homozygous dominant for that trait.

Mendel’s Dihybrid Crosses

Mendel performed another set of crosses in which he used peas that differed from each other in two traits rather than only one. The Law of Independent assortment is Mendel’s second law that states for different traits are inherited independently of each other.

Punnett Squares

Monohybrid Crosses Dihybrid Crosses

Section 10.2

Meiosis

Diploid and Haploid Cells

A cell with two of each kind of chromosome is called a diploid cell.

A cell containing one of each kind of chromosome is called a haploid cell.

Homologous Chromosomes

The two chromosomes of each pair in a diploid cell are called homologous chromosomes. Each pair of chromosomes has genes for the same traits, such as plant height.

Why Meiosis?When cells divide by

mitosis, the new cells have exactly the same number

and kind of chromosomes as the original cells. The

kind of cell division, which produces gametes

containing half the number of chromosomes as a

parent’s body cell is called meiosis. Meiosis occurs in

the specialized body cells of each parent that produce

gametes.

Phase One of Meiosis

During meiosis, a spindle forms and the cytoplasm divides in the same ways they do during mitosis. However, what happens to the chromosomes in meiosis is very different.

Phase Two of Meiosis

The second division in meiosis is simply a mitotic division of the products of meiosis I. The newly formed cells in some organisms undergo a short resting stage. In other organisms, however, the cells go from late anaphase of meiosis I directly to metaphase of meiosis II.

Meiosis Provides for Genetic Variation• Genetic recombination

-Each cell undergoing meiosis has seven pairs of chromosomes.

Genetic recombination is the reassortment of chromosomes and the genetic information

they carry, either by crossing over or by independent segregation of homologous chromosomes. Crossing over is the process

when chromatids pair so tightly that non-sister chromatids from homologous chromosomes can actually break and exchange genetic material.

• Meiosis explains Mendel’s results-The segregation of

chromosomes in anaphase I of meioses explains that each parent gives one allele for each trait at random to each offspring.

NondisjunctionThe failure of homologous chromosomes to separate properly during meiosis is called

nondisjunction. Polyploidy is an organism with more than the usual number of chromosome sets.

Gene Linkage and Maps

Genes sometimes appear to be

inherited together instead of

independently. If genes are close together on the

same chromosome, they usually are

inherited together.

Meiosis vs. Mitosis

THE END!

Finally.