Chapter 11 Introduction to Genetics Vocab genetics fertilization

Chapter 11 – Introduction to Genetics Vocab: genetics, fertilization, true-breeding, trait, hybrid, gene, allele, segregation, gamete, probability, Punnett square, homozygous, heterozygous, phenotype, genotype, independent assortment, incomplete dominance, codominance, multiple alleles, polygenic traits, homologous, diploid, haploid, meiosis, tetrad, crossing-over, gene map

11 -1 The Work of Gregor Mendel Genetics is the scientific study of heredity. Gregor Mendel • Austrian monk in what is now the Czech Republic in 1822 • Studied garden peas

Pea Plants • Fertilization is when male and female reproductive cells join. • Male cell = pollen; Female cell = egg

• Pea plants are normally self-pollinating – egg and pollen from same plant – one parent • True-breeding plants – after self-pollinating they have identical offspring • Mendel began manipulating the pollination – cross-pollination

• Mendel studied seven different traits that had two contrasting forms • Trait = a specific characteristic that varies from one individual to another

The offspring of crosses between parents with different traits are called hybrids. Mendel crossed plants with the two forms • P: Parental Generation: Tall x Short • F 1: First filial Generation: all Tall • F 2: Second filial Generation: 75% tall & 25% short

Made two conclusions: • Inheritance is determined by factors passed from one generation to the next called genes. Genes have different forms called alleles. • The principle of dominance states that some alleles are dominant and others are recessive. Recessive alleles are masked by dominant alleles.

• Segregation – When each F 1 plant flowers and produces gametes, the two alleles segregate from each other so that each gamete carries only a single copy of each gene. Therefore, each F 1 plant produces two types of gametes – those with the allele for tallness and those with the allele for shortness

Segregation

11 -2 Probability and Punnett Squares The likelihood that a particular event might occur is called probability. Coin landing on heads = 50% Coin landing on heads three times in a row = ½ x ½ = 1/8 The principles of probability can be used to predict the outcomes of genetic crosses.

Punnett squares can be used to predict and compare the genetic variations that will result from a cross. • Homozygous = organism with two identical traits; TT or tt • Heterozygous = organisms with two different traits; Tt • Phenotype = physical characteristics; tall or short • Genotype = genetic makeup; TT, Tt, or tt

Punnett Square Parent 1 genotype=TT Parent 2 genotype=tt

Punnett Square Parent 1 genotype=TT Parent 2 genotype=tt T t t Tt T

11 -3 Exploring Mendelian Genetics Independent Assortment = looks at the inheritance of two traits The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes. Independent assortment helps account for many genetic variations observed in plants, animals, and other organisms.

Beyond Dominant and Recessive Alleles • Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes. – Incomplete dominance – Codominance – Multiple alleles – Polygenic traits

1. Incomplete dominance – flower color (primrose, 4 o’clocks, snapdragon), horse coat color, sickle-cell anemia • 4 o’clocks: – RR= red – RW= pink – WW= white • Each dominant allele produces red pigment: RW makes ½ as much and appears pink

Incomplete Dominance

2. Codominance – both alleles contribute to the phenotype • Chickens – allele for black feathers is codominant with white feathers… heterozygote is an “erminette” speckled chicken • Human AB blood type

3. Multiple Alleles – more than two possible alleles exist in a population • Human ABO blood typing system • Coat color in rabbits

4. Polygenic Traits – traits controlled by two or more genes • At least three genes control pigment in the eyes of fruit flies • The range of skin colors is from more than four genes

Applying Mendel’s Principles • 1900’s American geneticist Thomas Hunt Morgan started studying fruit flies, Drosophila melanogaster because it was small, reproduce quickly, and have as many as 100 offspring

Characteristics of any organism are determined by interaction between genes and the environment including climate, soil, food availability, pollution.

11 -4 Meiosis • Chromosome number • Genes are located on chromosomes in the cell nucleus. • Chromosomes are found in pairs called homologous chromosomes. – Fruit flies have 8 chromosomes and 4 homologous pairs; 4 from female and 4 from male

• A cell with both sets of homologous chromosomes is diploid – Symbol = 2 N; body cells are diploid • A cell with one set of chromosomes is haploid – Symbol = N; gametes are haploid

Phases of Meiosis • Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half through the separations of homologous chromosomes in a diploid cell.

Terms • Tetrad = structure formed during meiosis in which homologous chromosomes pair up. • Crossing over = the 4 chromatids of a tetrad may exchange portions resulting in new combinations of alleles.

Mitosis results in the production of two genetically identical diploid cells, whereas meiosis produces four genetically different haploid cells.