Unit 4 Genetics

Ch. 11 Introduction to Genetics

The Work of Gregor Mendel

Genetics - the scientific study of heredity

Mendel is considered the “Father” of genetics

Gregor Mendel’s Peas

Mendel was an Austrian monk, that worked on pea plants

His experiments with pea plants laid the foundation of the science of genetics

Gregor Mendel’s Peas

Fertilization - when male & female sex cells join together

Pea flowers are normally self-pollinating, meaning the sperm cells in the pollen fertilize the egg cells in the same flower

Gregor Mendel’s PeasSeeds produced by self-pollination inherit all of their characteristics from the single plant that bore them

True-breeding - if plants were allowed to self-pollinate, they would produce offspring identical to themselves

Gregor Mendel’s Peas

Mendel wanted to produce seeds by joining male & female sex cells from 2 different plants

He cut off the pollen (male) parts of a plant, & dusted pollen from another plant onto the flower (female)

Gregor Mendel’s Peas

Cross-pollination - produces seeds that had 2 different parent plants

Genes & Dominance

P - parent generationF1 - First generation (first generation of offspring)

F2 - Second generation (offspring from the F1 generation)

Genes & DominanceTrait - a specific characteristic

Ex.) seed color, plant heightHybrids - offspring of crosses between parents with different traitsEx.) cross between plant with yellow seed color & plant with green seed color

Genes & Dominance

Mendel concluded that biological inheritance is determined by factors that are passed from 1 generation to the next

Genes - chemical factors that determine traits

Genes & Dominance

Alleles - different forms of a geneEx.) gene for plant height occurs in 1 form that produces tall plants & in another form that produces short plants

Mendel’s 2nd conclusion is the principle of dominance

Genes & DominanceThe principle of dominance states that some alleles are dominant & others are recessive

Dominant allele for a trait will always be exhibited (expressed or shown)

Recessive allele will only be expressed when a dominant allele is not present

Mendel’s Seven F1 Crosses on Pea Plants

Segregation

Gametes - sex cells (sperm or egg)

Segregation - during gamete formation, alleles segregate (separate) from each other so each gamete only carries a single copy of each gene

Segregation

Therefore, each F1 plant produces 2 types of gametes, those with the allele for tallness & those with the allele for shortness

Punnett Squares

Punnett square - a diagram that might result from a genetic cross

Punnett squares can be used to predict & compare the genetic variations that will result from a cross

Punnett SquaresHomozygous - organisms that have 2 identical alleles for a particular traitEx.) TT or tt

Heterozygous - organism that has 2 different alleles for the same traitEx.) Tt

Punnett Squares

Phenotype - physical characteristicsEx.) Tall plants

Genotype - genetic makeupEx.) TT

Independent AssortmentIndependent assortment - genes for different traits can segregate (separate) independently during gamete formation

Independent assortment increases genetic variation (genetic diversity, helps create genetically different organisms)

A Summary of Mendel’s Principles

1. The inheritance of biological characteristics is determined by individual units - genesGenes are passed from parents to their offspring

A Summary of Mendel’s Principles

2. In cases where 2 or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant & others may be recessive

A Summary of Mendel’s Principles

3. In most sexually producing organisms, each adult has 2 copies of each gene (1 from each parent)These genes are segregated (separated) from each other when gametes are formed

A Summary of Mendel’s Principles

4. The alleles for different genes usually segregate (separate) independently of 1 another

Beyond Dominant & Recessive Alleles

Some alleles are neither dominant nor recessive, & many traits are controlled by multiple alleles or multiple genes

Beyond Dominant & Recessive Alleles

Incomplete dominance - when 1 allele is not completely dominant over anotherThe heterozygous phenotype

is somewhere in between the 2 homozygous phenotypes

Beyond Dominant & Recessive Alleles

Codominance - where both alleles contribute to the phenotypeFlowers would not be pink, (a blend of red & white), but both red & white speckled

Beyond Dominant & Recessive Alleles

Multiple alleles - when genes have more than 2 allelesIt does not mean that an individual can have more than 2 alleles

It only means that more than 2 possible alleles exist in a population

Multiple Alleles

Beyond Dominant & Recessive Alleles

Polygenic traits - traits controlled by 2 or more genesEx.) at least 3 genes are responsible for making the reddish-brown pigment in the eyes of fruit flies

Genetics & the Environment

The characteristics of any organism are not determined solely by the genes it inherits

Characteristics are determined by interaction between genes & the environment

Genetics & the Environment

Ex.) genes may affect a sunflower plant’s height & the color of its flowers

However, these conditions are also influenced by climate, soil conditions, & the availability of water

Ex.) Rabbit fur color in winter & summer

Chromosome NumberAll cells of an organism (except for sex cells, gametes) have the same # of chromosomes

Each body cell has 2 sets of chromosomes

Homologous chromosomes - the same chromosomes, 1 set from each parent

Homologous Chromosomes

Chromosome Number

Diploid - (2n) - a cell that has both sets of homologous chromosomes

Haploid - (n) - a cell that has half the normal set of chromosomes, or 1 set (only sex cells are haploid)

Phases of Meiosis

Meiosis - process of reduction division, where the # of chromosomes per cell is cut in 1/2, through the separation of homologous chromosomes in a diploid cell

Phases of Meiosis

During meiosis 1, crossing-over may occur

Crossing-over - when chromosomes exchange portions of their chromatids

Phases of MeiosisCrossing-over results in the exchange of alleles between homologous chromosomes & produces new combinations of alleles

Crossing-over increases genetic variation (genetic diversity, helps create genetically different organisms)

Phases of Meiosis

Meiosis II, begins with 2 genetically different haploid (n) cells, & results in 4 (n) genetically different haploid cells

Therefore, Meiosis II is a mitotic division

Meiosis I

Meiosis II

Gamete Formation

In males, the haploid gametes are sperm

In females, the haploid gametes are eggs

Comparing Mitosis & Meiosis

Mitosis results in the production of 2 genetically identical diploid (2n) cellsMitosis produces all cells of the body, except sex cells

Meiosis produces 4 (n) genetically different haploid cellsMeiosis produces ONLY sex cells (gametes)

Gene Linkage

Each chromosome is a group of linked genes

It is the chromosomes, however, that line up independently, not individual genes (Principle of Independent Assortment)