Chromosomes & Meiosis Unit 5: Mendelian Genetics

Chromosomes & Meiosis

Unit 5: Mendelian

Genetics

Learning Goals1. Define homologous chromosomes and explain diploid and haploid.2. Describe the overall process of meiosis.3. Explain crossing over including the step in meiosis where it takes place and why it is important.4. Describe the two types of gamete formation and how fertilization occurs.

Chromosome Structure

Made of DNA wrapped around histones (proteins)Sections of DNA make up genes

Homologous Chromosomes

Each body cell has 22 sets of chromosomes Homologous ChromosomesHomologous Chromosomes = a

pair of chromosomes that have the same genes at the same locations

1 came from mom 1 came from dad

From mom

From dad

Homologous chromosomes

Chromosome NumberDiploid (2n)Diploid (2n): cells that have 2 sets of homologous chromosomes (2n)

Human body cells are diploid (4646 chromosomes or 23 pairs)

Haploid (n): cells that have 1 set of chromosomes (n)

sex cells (sperm & egg) are haploid (23 chromosomes total)

1) A bug has a haploid number n=5. What is the diploid number (2n)?

2) A crocodile has a diploid number 2n=50. What is the haploid number (n)?

meiosismeiosis

Haploid gamete

Diploid

Haploid gamete

2n

n n

Karyotypes

A karyotype shows an individual’s complete set of chromosomes Autosomes = Chromosome #1-22 Sex chromosomes = Chromosome #23

XX = Female XY = Male

Karyotypes are studied to determine irregularities in the chromosomes and can show genetic disorders.

Human Female Karyotype

Human Male Karyotype

Down Syndrome Female

Trisomy

On the karyotype you were given circle and label the following:

Autosomes Sex chromosomes 1 set of homologous chromosomes

*Determine and label the gender!

MeiosisMeiosis

Cell division that results in the formation of gametes: Gametes = Sex Cells (sperm & eggs)

Human gametes have 23 chromosomes (22 autosomes + 1 sex chromosome)

23 Egg23

Steps of Meiosis

InterphaseDNA is copied before the start of Meiosis I.This makes 2 identical sister chromatids

Meiosis 1

Four steps: Prophase 1 Metaphase 1 Anaphase 1 Telophase 1

Prophase 1

The homologous pairs of chromosomes line up side by side. (tetrad = 4)

Crossing-overHomologous chromosomes can Cross-over and exchange a piece of the chromosome randomly

Crossing-over is very important because it creates genetic variation (new combinations of genes), which makes us all different. This is why you do

not look exactly like your siblings.

Crossing over happens randomly.However, some genes are linked, or close together on the chromosome, meaning they are more likely to be inherited together.

Red hairFreckles

Chromosome 16

After crossing-over, the homologous chromosomes separate & the cell divides

Meiosis II

Four steps: Prophase 2 Metaphase 2 Anaphase 2 Telophase 2

Sister chromatids separate & the cells divide.Result is 4 genetically different haploid daughter cells

Meiosis Overview

Gametes produced during meiosis are needed for sexual reproductionCells (diploid) divide twice resulting in 4 daughter cells (haploid)Each cell has half the number of chromosomes as the original cellEach new cell is genetically different

Mendel’s Law of Segregation

Genes segregate (separate) into different cells during meiosis

Each gamete only gets one possible gene

Principal of Independent Assortment

Genes for different traits can segregate (separate) independently during the formation of gametes.

The inheritance of one trait is not determined by the inheritance of another trait

Ex: The gene for eye color is not connected to the gene for hair color.

Independent assortment and crossing-over account for the many genetic variations observed in plants, animals, and other organisms. Assume there was only one cross-over in a

man’s sperm and one cross-over in a woman’s egg, they have the possibility of producing 4,951,760,200,000,000,000,000,000,000 genetically different children

SpermatogenesisIn males meiosis occurs in the testes

produces 4 sperm

23

23

23

23

Spermatogenesis

OogenesisIn females meiosis occurs in the ovaries

produces only 1 egg and 3 polar bodies

Egg23

Polar Bodies

Oogenesis

Fertilization: the combining of sperm and egg to form a zygote

Zygote

46

2323

First cell of a new organism

Only one sperm and one egg come togetherThe first sperm to reach the egg will be the one to fertilize it.

Learning Goals1. Define homologous chromosomes and explain diploid and haploid.2. Describe the overall process of meiosis.3. Explain crossing over including the step in meiosis where it takes place and why it is important.4. Describe the two types of gamete formation and how fertilization occurs.