Chapter 10 Mendel and Meiosis Objectives: Analyze the the results obtained by Gregor Mendel in his experiments with garden peas. Predict the possible offspring of a genetic cross by using a Punnent square.

Standard 4. apply the principles of Mendelian inheritance to make predictions about offspring. identify dominant and recessive traits, given the results of a monohybrid cross in a scenario. distinguish between dominant and recessive traits, given the results of a monohybrid cross.

determine the genotype and phenotype of a monohybrid cross, given a Punnett square. diagram and analyze a monohybrid cross, given a genetic problem.

Father of Genetics— Gregor Mendel

Section 1 Mendel carried out the first important studies of pea plants, the passing on of characteristics from parents to offspring. Genetics is the branch of biology that studies heredity. Characteristics that are inherited are called traits.

Mendel used the pea plants to study genetics because they reproduce sexually with two distinct cells. Sex cells are called gametes. Ways Mendel carefully controlled his experiments and the peas he used were: He studied only one trait at a time to control variables and his plants were true-breeding plants.

A hybrid is the offspring of parents that have different forms of a trait, such as tall and short. His first experiments are called monohybrid crosses. First generation TT X tt Second generation Tt x Tt

In every case, Mendel found that one trait of a pair disappeared in the F1 generation only to reappear in the next generation. Some traits Mendel chose for his experiments were: seed shape, seed color, flower color pod color, plant height. Each organism has two factors that control each of his traits.

We call these different gene forms alleles..An organism’s two alleles are located on different copies of a chromosome—inherited from the female parent and the male parent. Mendel called the observed trait the dominant trait and the disappeared trait recessive. An upper case letter is used for the dominant trait and a lower case letter is used for the recessive trait. The dominant allele is written first.

Two alleles for each trait must separate when a zygote is formed. A parent, therefore, passes on at random only one allele for each trait to each offspring. The way an organism looks and behaves is called its phenotype. The phenotype of a tall plant is tall whether it is TT or Tt.

The gene combination an organism contains is known as its genotype.The genotype of a tall plant that has two alleles for tallness is TT or Tt. An organism is homozygous for a trait if its two alleles for the trait are the same. Which would be written TT or tt For tall or short plants.

An organism is heterozygous for a trait if its two alleles for the trait differ from each other. Which would be written Tt. Punnett Squares- A short hand way of finding the expected proportions of possible genotypes in the offspring of a cross.

This Punnett Square shows how we can diagram the genes. The orange bird has two dominant Agenes. We put two A s along the top of the square. The blue bird has two recessive a genes. We put two a s down along the left side of the square.

All the offspring have the genes Aa. They will all have orange feathers, but will carry a recessive gene for blue feathers. This is called the F1 generation.

Now suppose that two individuals from the F1 generation become parents. Here they are! The baby birds are called the F2 generation. You can see how their genes work out. The offspring are coded in the squares. One bird will be orange with two AA genes. Two birds will be orange with genes coded Aa. One bird will be blue and will have two recessive aa genes. Individual nests of birds may not turn out exactly like this, but if there are many baby birds, they will work out genetically with the ratios 1:2:1.

Dominant B and recessive b Cross between Heterozygous (Bb) parents In this case, the probablity of a child having bb is ?

If you know the genotypes of the parents, you can predict the possible genotypes of their offspring. Monohybrid crosses. Give phenotype and genotype. Cross Tt x Tt Cross TT x tt Cross Tt x tt

What kind of cross is this? Dihybrid Cross

A cross involving two different traits is called a dihybrid cross. Mendel took true-breeding pea plants with round and yellow seeds and crossed them with true breeding green and wrinkled seeds. Look at this cross and see results.

In the F1 generation he found all seeds to be heteorzygous round and yellow seeds.

He let the F1 generation plants to pollinate themselves. Show this cross. Mendel’s second law states that genes for different traits are inherited independently of each other. It is known as law of independent assortment.

Section 2 Meiosis Objectives: Analyze how meiosis maintains a constant number of chromosomes within a species. Infer how meiosis leads to variation in a species. Relate Mendel’s laws of heredity to the events of meiosis

Organisms have tens of thousands of genes that determine individual traits. In the body cells of animals and most plants, chromosomes occur in pairs.

A cell with two of each kind of chromosomes is called a diploid cell and is said to contain a diploid, or 2n, number chromosomes. Organisms produce gametes that contain one of each kind of chromosome, a cell with one each kind of chromosome is called a haploid cell and is said to contain a haploid or n.

Mendel’s conclusion was that parent organisms give one factor, or allele, for each trait to each of their offspring.

Meiosis A human body cell contains 23 pairs of chromosomes. The gametes - sperm or eggs - contain half this number of chromosomes, which is why meiosis is sometimes called 'reduction division' chromosomes

During prophase I homologous chromosomes pair and form synapses, a step unique to meiosis.

Metaphase I chromosomes (four chromatids) align at the metaphase plate.

Anaphase I Chromosomes, each with two chromatids, move to separate poles. Each of the daughter cells is now haploid (23 chromosomes), but each chromosome has two chromatids.

Telophase I Nuclear envelopes may reform, or the cell may quickly start meiosis II.

A human cell has 46 total or 23 pairs of chromosomes. Following mitosis, the daughter cells would each have a total of ______ chromosomes. After meiosis I, the two daughter cells would have _____chromosomes, and after meiosis II ______ chromosomes. A.46, 46, 46B.46, 23, 23 C.23, 23, 23D.46, 12, 1246, 46, 4646, 23, 23 23, 23, 2346, 12, 12

The two chromosomes of each a pair in a diploid cell help determine what the individual organism looks like. The paired chromosomes are called homologous chromsomes.

Why meiosis? When cells divide by mitosis, the new cells have exactly the same number and kind of chromosomes as the original cells. Meiosis occurs in body cells of each parent that produce gametes. Meiosis consists of two separate divisions. By the end of meiosis II, there are four haploid cells Male gametes -- sperm Female gametes -- egg Fertilized egg --zygote Fusion of haploid sex cells is called sexual reproduction.

Crossing over- exchange of genetic material. Cells that are formed by mitosis are identical to each other and to the parent cell. Meiosis however, provides genetic variation. The reassortment of chromosomes and the genetic information they carry, either by crossing over or by independent segregation of homologous chromosomes, is called genetic recombination. It is a major source of variation among Organisms.

Mistakes in Meiosis The failure of homologous chromosomes to separate properly during meiosis is called nondisjunction. Trisomy- extra chromosome Monosomy- lacks a chromosome Triploidy- Gamete with an extra set of chromosome