Meiosis: 10th Grade

Resources

Homework for Monday

Create a document which outlines the process of meiosis Your document must be accurate, must demonstrate knowledge, and must be creative• Label AT LEAST: gene; chromosome; DNA, crossing

over and all the steps of meiosis

What is the biological purpose

of sex?

Meiosis is a two step process

• Meiosis 1 separates the homologous pairs of chromosomes: this is the reduction division

• Meiosis 2 separates the sister chromatids (resembles mitosis)

• One single diploid cell produces 4 non-identical haploid cells

Essential definitions: (sister) chromatids

• Chromatid: one of two identical halves (sister chromatids) of a replicated chromosome

• NB: following crossing over, sister chromatids are no longer identical!

The process of pairing is called synapsis

Getting to grips with meiosis

• Meiosis – the movie• Short summary• Fabulous website based tutorial• Step through from Sumanas

Diploid germ cell chromosomes duplicate during S phase BEFORE meiosis

Homework for Monday

Create a document which outlines the process of meiosis AND explains one disease that is caused by an error during meiosis• Your document must be accurate, must demonstrate

knowledge, and must be creative• Label AT LEAST: gene; chromosome; DNA, crossing

over and all the steps of meiosis• Provide a summary of ONE disease that results from

an error during meiosis (a non-disjunction disorder)

Meiosis is a two step process

• Meiosis 1 separates the homologous pairs of chromosomes: this is the reduction division

• Meiosis 2 separates the sister chromatids (resembles mitosis)

• One single diploid cell produces 4 non-identical haploid cells

Essential definitions: homologous chromosomes

• Human somatic cells have 22 homologous chromosome pairs and one non-homologous pair

• One maternal/ one paternal• Same size and structure• Same genes at same loci• Alleles may differ

Essential definitions: (sister) chromatids

• Chromatid: one of two identical halves (sister chromatids) of a replicated chromosome

• NB: following crossing over, sister chromatids are no longer identical!

Crossing over and recombination

• homologous (duplicated) chromosomes associate during prophase 1 of meiosis

• The process of pairing is called synapsis

• Watch in detail here

Meiosis and genetic variation

Problems during meiosis

Non-disjunction

Production of gametes with an abnormal chromosome number

Down’s Syndrome (Trisomy 21)• Ten things people with Down’

s Syndrome would like you to know

• Most commonly due to non-disjunction in the female gamete during anaphase I/II Characteristic facial features

• Characteristic beautiful, happy nature

• Wide range of severity of disability

Identifying chromosomes (karyotyping)

Chromosomes are lined up in their homologous pairs according to:• Size (1 largest – 22 smallest)• Shape – position of the

centromere• Specific banding (staining)

patterns using dyes (Giemsa)More later!

• and in more detail here

Genetics and Inheritance

• Page 197 – 204

Essential vocabulary of genetics:

• Gene: A length of DNA that is the unit of heredity• Genotype: The genetic make-up of an organism,

in terms of the alleles present• Allele: Any of two or more alternative forms of a

gene• Phenotype (trait): Physical or other features of

an organism. Caused by combination of genotype AND environment

• Autosome: Any chromosome which is not a sex chromosome

Essential definitions: homologous chromosomes

• Human somatic cells have 22 homologous chromosome pairs and one non-homologous pair

• Each chromosome in a homologous pair contains genes for the same characteristic in the same position

• One maternal/ one paternal• Same size and structure• Same genes at same loci• Alleles may differ

Each cell has (at least) 2 genes for any characteristic

Homozygous Homologous chromosomes have identical alleles of a particular gene. Two identical homozygous individuals that breed together will always have the same characteristics ‘Pure breeding’/’wild type’

Heterozygous: Chromosomes have two different alleles of a particular gene (Ff).•‘not ‘’pure breeding’…

Alleles

Dominant and recessive alleles• A dominant allele (e.g. F)

will be expressed if it is present

• A recessive allele (e.g. f) will only be expressed when there is no dominant allele of the gene present

• Codominant alleles (eg IA and IB) are both equally expressed

Some ‘celebrity genes’

• BRAC1 and 2

Celebrity genes (2)

Mutations in the CTFR gene cause cystic fibrosis

Celebrity genes (3)

APOE4 increases the risk of Alzheimer’s

Disease

Working out genetic crosses

1. Genetic diagrams (Punnet Squares)2. Pedigree Charts

Working out single trait inheritance (monohybrid crosses)

Monohybrid cross: a mating between

two individuals who have different alleles at one gene

locus

Working out single trait inheritance

• Is the gene located on an autosome (chromosome 1 – 22 in humans) or on a sex chromosome (X/Y- in which case inheritance is sex-linked)

• Does the gene have 2 or more than two alleles?

• Is the pattern of gene expression dominant/recessive or co-dominant?

Working out monohybrid crosses

Always list:1. EACH parent’s phenotype (e.g. green pea/

cystic fibrosis free)2. EACH parent’s genotype (2 alleles for each

parent)3. 2 gametes for each parent4. F1 generation genotypes (and proportions)5. F1 generation phenotypes (and proportions)

An example: cystic fibrosis

• An explanation of the disease• Life with cystic fibrosis

Cystic fibrosis is disease associated with expression of a recessive gene

Convention for working out monohybrid crosses

Working out inheritance: Punnet Squares

A chart that shows all the possible combinations of alleles that can result from a genetic crossIntroduction to Punnett Squares

Tt

Steps in Making a Punnett Square1. Draw a square and divide it into four.2. The alleles for one parent go on the top. The alleles for the other parent go down the side.3. The boxes are like a multiplication problem, with one allele contributed by each parent.

I can only give 1/2 of my DNA to my offspring, so I

will either give my baby planta T OR a t.

One Trait Inheritance • T = tall, t = short• Complete the punnet

square in your notes.• Label the P gametes• Label the F1

generation• What is the

phenotypic ratio?• What is the

geneotypic ratio?

One Trait Inheritance

TT Tt

Tt tt

P gametes

F1 generation

Phenotypic Ratio = 3:1

Genotypic Ratio = 1:2:1

Cystic fibrosis is disease associated with expression of a recessive gene

Cystic fibrosis Punnet Squares (1)

Cystic fibrosis Punnet Squares (2)

Working out inheritance: Genetic Diagrams

Genetic diagrams

Pedigree Charts

• A simple introduction

CODOMINANCE

Some genes have alleles that are both expressed in the heterozygote individuals

Codominance

• Both alleles are equally dominant!

• Both alleles are expressed fully, resulting in a new phenotype!

• e.g. Roan horses/cattle – red and white hairs are intermingled

Multiple alleles

Multiple alleles

• When Mendel studied pea ‘traits’, there were only 2 alleles for each of the traits he studied: (round/wrinkled, tall/short, yellow/green)

• In fact, you CAN have > 2 alleles for any trait• Human blood types are an example of a trait

with multiple alleles (A, B, O)• Human blood types are also an example of

codominance

Human Blood types

• Type O is recessive• Types A and B are

codominant • Both Types A and B

are dominant to Type O

What does ‘blood type’ mean?

We have both antigens (markers) and antibodies in our blood

• 45% are Type O• 42% Type A

• 10% are type B• 3% are type AB

What happens if you receive the wrong type of blood in a transfusion?

Sex linked genes

Why are some characteristics ‘sex linked’?

• The X-chromosome (23) carries 1098 genes

• Over 100(recessive) genes for genetic disorders have now been mapped to the X-chromosome 23.

• The human Y-chromosome is much smaller, and only carries a few genes

Why are sex-linked diseases more common in males?

1. Males have only 1 X chromosome; thus all X-linked alleles will be phenotypically expressed in males, even if they are recessive…

2. Females must have BOTH copies of the allele to be recessive, in order to express the gene phenotypically

3. Thus the recessive phenotype of a sex-linked genetic disorder is much more common in males

4. Sex-linked genes can move from fathers to their daughter, where the daughters will be unaffected carriers, but pass the gene to their sons…

Sex-linked inheritance

Red-green colour blindness

• test......

Genetics of colour blindness• 0.4% of women are colour

blind• 7% of men are colour blind• In colour blindness, green

or red cones work poorly, or not at all…

• Genes for green and red cones are close to each other on the X-chromosome

• Why is ‘blue blindness’ rare?

Haemophilia• Recessive, sex linked disease

which causes abnormal blood clotting

• Individuals are at risk of severe bleeding

• Many die from brain haemorrhages

• Type A: Lack of Factor VII: 80% of cases of haemophilia

• Type B: Lack of Factor IX: 20% of cases

Haemophilia in the Royal families of Europe

• Queen Victoria has 9 children (4 sons, 2 daughters). She was a carrier for the Haemophilia gene

• 1 of 4 sons developed haemohilia (died at age 30)• 2 of 5 daughers were carriers (married and moved to Spain,

Russia, Germany)

How did Mendel determine the genotype of an ‘unknown’ individual plant?