Meiosis, Genetics and Heredity

Meiosis – A Source of DistinctionMeiosis – A Source of Distinction

Why do you share some but not all characters of each parent?

What are the rules of this sharing game?

At one level, the answers lie in meiosis.This makes for a lot This makes for a lot of genetic variation. of genetic variation. This trick is This trick is accomplished accomplished through through independent independent assortmentassortment and and crossing-overcrossing-over..

Genetic VariationCrossing over – chromosomal segments are exchanged between a pair of homologous chromosomes.

Independent Assortment-random distribution of alleles occurs during gamete formation.

Sexual Reproduction v. Asexual

Reproduction

Genetic Variation occurs in Asexual or Sexual Reproduction?

Meiosis is

Reduction Division

Chromosome number is cut in half by separation of homologous chromosomes in diploid cells

Differences BetweenMitosis & Meiosis

• MITOSIS-Body Cells produces 2 identical diploid (2n) cells

• MEIOSIS-Sex Cells produces 4 different haploid (n) cells

Body CellsSomatic Cells

Sex Cells

Diploid: containing two complete sets of chromosomes, one from each parent.

Haploid: is the term used when a cell has half the usual number of chromosomes.

Chromosome Numberin Body Cells vs. Gametes

• Body Cells (Mitosis)Diploid (2n) – results in 2 genetically identical diploid cells.

Gametes (Meiosis)Haploid (n) – results in 4 genetically different haploid cells.

Human Cells 2n=46.

• Sperm Cell– 23 chromosomes– Gamete is haploid (n)

• Egg Cell– 23 chromosomes– Gamete is haploid (n)

• White Blood Cell– 46 chromosomes– Body cell is diploid (2n)

Sex CellSex CellGametesGametes

Body CellsBody CellsSomatic CellsSomatic Cells

KaryotypingNotes and Activity Pages

Karyotyping Activity

Introduction to Genetics

Genetics = the study of heredity

• Gregor Mendel–Principle of Dominance –Law of Segregation–Law of Independent Assortment –The Father of Genetics–carried out important studies of heredity

• Heredity—the passing on of traits from parents to offspring

• He studied pea plants

• Chromosomes–Tightly coiled DNA

where your genes are carried

• Genes–Sections of DNA that control your

traits

• Homologous Chromosomes–Chromosomes that are the same size and

shape and carry the same genes. You get one from your mother and one from your father that match up.

Short pea plant Tall pea plant

All tall pea plants

3 tall: 1 short

P1

F1

F2

–Alleles (Genotypes)– The different forms a gene can take. (Letters)

• 2 Types of Alleles:–1. Dominant - Represented by a capital letter

»(Exp. A)

–2. Recessive - Represented by a lower-case letter (Exp. a)

• Trait- a specific characteristic that is controlled by genes (Genotypes)– Exp. Eye color, hair color, height

–Genotype• The combination of two alleles (letters) that

represent the genetic make-up of an individual. (AA, Aa, aa) (Alleles)

–Phenotype• The observable physical characteristic of an

individual that are the result of its genotype. (Long eyelashes, Blue or Brown eyes) (Trait)

3 Types of Genotypes:

• 1. Homozygous Dominant– 2 identical alleles that are capitalized (AA, TT)

• 2. Heterozygous– 2 different alleles (Aa, Tt)

• 3. Homozygous Recessive– 2 identical alleles that are lower-case (aa, tt)

Principle of Dominance

• An organism with a recessive allele for a particular trait will exhibit that form only when the dominant allele for the trait is not present.

• Organisms with a heterozygous genotype (Aa) will never exhibit the recessive trait because it is hidden (masked) by a dominant allele.

• You try this one:

A A

A

a

Monohybrid Crosses PracticeWorksheet

Dihybrid Cross:a cross that shows the possible offspring for two traits

16 squares

Fur Color:B: Black b: White

Coat Texture:R: Roughr: Smooth

In this example, we will cross a heterozygous individual with

another heterozygous individual. Their genotypes will be:

BbRr x BbRr

Dihybrid Cross

BbRr x BbRr

First, you must find ALL possible gametes that can be made from each parent.

Remember, each gamete must have one B and one R.

Dihybrid CrossBbRr x BbRr

Possible gametes:BRBrbRbr

Next, arrange all possible gametes for one parent along the top of your Punnett Square, and all possible gametes for the other parent down the side of your Punnett Square…

Dihybrid Crosses:a cross that shows the possible offspring for two traits

Fur Color:B: Black b: White

Coat Texture:R: Roughr:

Smooth

BbRr x BbRr

BR

bR

br

bR

Br

BR

br

Br

Then, find the possible genotypes and phenotypes of the offspring

Dihybrid Crosses:a cross that shows the possible offspring for two traits

Fur Color:B: Black b: White

Coat Texture:R: Roughr:

Smooth

BbRr x BbRr

BR

bR

br

bR

Br

BR

br

Br

BBRR

BbRR BbRr

BBRr BBrr BbRr Bbrr

BbRR BbRr bbRR bbRr

BbRr Bbrr bbRr bbrr

BBRr

BR bR

br

bR

Br

BR

br

Br

BBRR

BbRR

BbRr

BBRr BBrr BbRr Bbrr

BbRR

BbRr bbRR

bbRr

BbRr Bbrr bbRr bbrr

BBRr

How many of the offspring would have a black, rough coat?

How many of the offspring would have a black, smooth coat?

How many of the offspring would have a white, rough coat?

How many of the offspring would have a white, smooth coat?

Fur Color:B: Black b: White

Coat Texture:R: Roughr:

Smooth

BR bR

br

bR

Br

BR

br

Br

BBRR

BbRR

BbRr

BBRr BBrr BbRr Bbrr

BbRR

BbRr bbRR

bbRr

BbRr Bbrr bbRr bbrr

BBRr

How many of the offspring would have black, rough coat?

How many of the offspring would have a black, smooth coat?

How many of the offspring would have a white, rough coat?

How many of the offspring would have a white, smooth coat?

Fur Color:B: Black b: White

Coat Texture:R: Roughr:

Smooth

Phenotypic Ratio

9:3:3:1

Dihybrid Crosses PracticeWorksheet

Mendelian Exceptions- Not all genes show simple patterns of dominant and recessive alleles

• Because the majority of traits are controlled by more than one gene/alleles

• There are some exceptions to Mendel’s Laws.

1. INCOMPLETE DOMINANCE

• Neither allele is completely dominant

• Both alleles combine equally to give a new trait.

• Called hybrids• EX: red flower (RR) crossed

with white flower (R’R’) produces pink flower (RR’)

Incomplete DominanceIn snapdragons, flower color is controlled by incomplete dominance. The two alleles are red (R) and white (W). The heterozygous genotype is expressed as pink.a)What is the phenotype of a plant with the genotype RR? ___________b)What is the phenotype of a plant with the genotype WW? ___________c)What is the phenotype of a plant with the genotype RW? ___________ 

• Which of the following would be the correct product from a cross between two heterozygous pink snapdragons? –A 1 red, 2 pink, 1 white–B 2 red, 2 white–C 2 red, 1 pink, 1 white–D 1 red, 1 pink, 2 white

2. CODOMINANCE• Both alleles are

expressed• EX: cross a black

chicken with a white chicken, offspring will be black and white checkered. Roan cow (RW)

Red cow (RR) White cow (WW)

In some chickens, the gene for feather color is controlled by codominance. The allele for black is B and the allele for white is W. The heterozygous phenotype is known as erminette (black and white spotted).a)What is the genotype for black chickens? ____b)What is the genotype for white chickens? ____c)What is the genotype for erminette chickens? ____ Two erminette chickens were crossed. Show the Punnett square.a)What’s the probability they would have a black chick? ____%b)What’s the probability they would have a white chick? ____% 

• In a certain cactus, prickly spines can be two pronged or one pronged. If a true breeding (homozygous) one-pronged cactus is crossed with a true breeding two-pronged cactus, the F1 generation has a mixture of spines, some are two-pronged, some are one-pronged.

• Is this an example of codominance or incomplete dominance?

codominance

Practice Problems

MULTIPLE ALLELES: more than two alleles control a phenotype

• Ex: blood type

Genotypes:AO- 2/4 or 50%OO- 2/4 or 50%

Phenotypes:Type A- 2/4 or 50%Type O- 2/4 or 50%

Blood type inheritance

• Blood type = presence or absence of proteins on red blood cells

• Usually have to do more than one punnett square to determine possibilities for kids.

PhenotypPhenotypeses

Blood Blood typestypes

GenotypeGenotypess

Alleles for Alleles for blood blood typetype

AA AA or AOAA or AO

BB BB or BOBB or BO

ABAB Only ABOnly AB

OO Only OOOnly OO

Human blood types are determined by genes that follow the CODOMINANCE (multiple alleles) pattern of inheritance. There are two dominant alleles (A and B) and one recessive

allele (O). 1.Write the genotype for each person based on the description: 

• Homozygous for the “B” allele______

• Heterozygous for the “A” allele______

• Type O ______• Type “A” and had a type “O” parent

______• Type “AB” ______

 Pretend that Brad Pitt is homozygous for the type B allele, and Angelina Jolie is type “O.” What are all the possible blood types of their baby?

• Traits controlled by genes located on sex chromosomes are called sex-linked traits.

• The alleles for sex-linked traits are written as superscripts of the X or Y chromosomes.

Sex-linked inheritanceSex-linked inheritance

Sex DeterminationSex Determination• If you are female,

your 23rd pair of chromosomes are homologous, XX.

• If you are male, your 23rd pair of chromosomes XY, look different.

X XFemale

YXMale

XX Female

XY Male

X

X

X Y

XX Female

XY Male

XX Female

XY Male

Sex Linked Traits• When genes are located on the X chromosome,

females receive two alleles for these genes, but males only receive one.

• In males, the genotype is automatically known.– Colorblind male - XcY (recessive)– Normal male - XCY (dominate).

• A female can be:– XCXC - normal– XCXc - carrier– XcXc - colorblind

4. SEX-LINKED TRAITS: controlled by genes located on sex chromosomes

• Usually carried on X chromosome• Since females are XX, they are

usually carriers of the trait• Since males are Xy, they have one

big & one small, stumpy chromosome. • The small chromosome (y) does

not carry an allele so whatever allele is on the X (donated by mom) is what the boy will have.

• He is either completely dominant or completely recessive

• Boys cannot be a carriers of a sex linked trait.

• Males can pass it to all of their daughters, none to sons

• Females have 50/50 chance of passing it to all of their children

• Ex: – Hemophilia- can’t clot blood– Colorblindness- can’t see

certain colors.

H= normal blood clotting h=hemophilia   A woman that is heterozygous for normal blood clotting is cross with a man that has normal blood clotting. What are the chances that their offspring will have hemophilia? 

5. Polygenic inheritance-2 or more genes affect the phenotype.

• Ex: height, weight, skin color, eye color

Heterochromia- uneven distribution of pigment resulting from disease or injury

PEDIGREE CHARTS

A family history of a genetic condition

What is a pedigree chart?

• Pedigree charts show a record of the family of an individual

• They can be used to study the transmission of a hereditary condition

• They are particularly useful when there are large families and a good family record over several generations.

Organising the pedigree chart• Individuals in each generation are identified by Arabic

numerals numbered from the left• Therefore the affected individuals are II3, IV2 and IV3

I

II

III

IV

© 2007 Paul Billiet ODWS

Worksheet