Human Genetics

Chapter 7

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The Role of ChromosomesA. Chromosome number

1. Each human sperm/egg has 23 chromosomes

2. Each human body cell has 23 pairs of chromosomes

3. Each type of organism has a different number of chromosomes.

Humans - 46 chromosomes

Fly – 8 chromosomes

Cat – 38 chromosomes

23 23

23 PAIRS

The Role of Chromosomes

Chromosome number- The last pair of chromosomes,or the 23rd pair are called the sex chromosomes.- Males have an X and a Y chromosome (XY)- Females have two Xchromosomes (XX)

- Autosomes do not determine aperson’s gender. They are body chromosomes. They determine ones characteristics!

- Chromosome #1-22 3

• X & Y Chromosomes& Traits

• X chromosome much larger than the Y

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Amniocentesis, a sampling of the fetal fluid which then can help determine if there are any genetic disorders of the developing fetus.

Geneticist will look at the chromosomes of the fetus.

A way to tell chromosome number

Karyotype

Human TraitsA. Incomplete dominance, neither allele/gene is completely

dominant nor completely recessive.

- Heterozygous phenotype is in between the two homozygous phenotypes

- individual is a blend

Snap dragons (flowers)RR= Redww= White

If a red snap dragon were crossed with white snapdragon, the heterozygous plant would be PINK!

RR x ww

Offspring = Rw = pink6

B. Codominance- Codominant alleles/genes are both fully and separately expressed.

- Both phenotypes are shown in the Heterozygous individual.

- Black hamster crossed with a white hamster- BB X WW

- The heterozygous

individual BW, would

be both black and white

in color.7

B B

W

W

BW

BW

BW

BW

Co-dominance and Multiple Alleles in Blood Types

Example: Blood Types

The four blood types are Type A, Type B, Type AB, Type O

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The 3 multiple alleles are A, B, and O

• A and B are always dominant to O.– AO = A type blood– BO = B type blood

• A and B are NOT dominant to each other, they are Codominant

So remember… There are 3 alleles (A, B, O), however there are 4 blood types. (A, B, AB, O)

Genes Blood type

OO O type

AB AB type

AA or AO A type

BB or BO B type

Cross a woman Heterozygous for A-blood and a man with AB-blood

–Parent genotypes:

–Genotypes:

–Phenotypes:

A

OA

B

AA

BO

AO

AB

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AO x AB

1AA:1AB:1AO:1BO

1: AB type2: A-type1: B-type

More Codominance

Sickled RedBlood Cell•African American

Population•Resistance to Malaria

Normal RedBlood Cell

Sickle Cell • NN= Normal Blood Cells

• SS= Sickled Blood Cells

• NS = Both Normal and Sickled Cells

• Cross a heterozygous mom with a heterozygous dad for blood cells.

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Parent genotypes: Genotypes: Phenotypes:

NOW YOU TRY: Bb x Bb as Complete, Incomplete, & Co- Dominance

Say that:B = Black

b = White

W = White

Complete Dominance

Incomplete Dominance

Co-Dominance

Genotype Ratio

Phenotype Ratio

1BB:

2Bb:

1bb

1BB:

2Bb:

1bb

1BB:

2BW:

1WW

3 Black :

1 White

1 Black :

2 Gray :

1 White

1 Black:

2 Black & White:

1 White

Polygenic Traits• Polygenic traits are produced by two or more

genes.

• This occurs when many genes interact to produce one trait.

• Example: Eye Color, skin color, height

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C. Sex-linked traits - Genes on the X chromosome

1. Only the X Chromosome carries genes

so if a gene is on the X chromosome, the female would have TWO of those genes

and the male would have only ONE.

X Y

X X

Female that has the disorder Female carrier for a disorder

Male that has the disorder

C.Sex-linked traits- A genetic disorder that is found or linked to the X

chromosome ONLY- Females can carry a sex-linked genetic disorder and

not have the disorder.- This is known as being a carrier of the disorder.- In order for the female to have the disorder both X

chromosomes must have the gene for the disorder

- Males (XY) express all of their sex linked genes because they only have one X chromosome.

- If the X chromosome has the gene, the male has the disorder.

- Males can not be carriers because they only have one X chromosome

- Examples: Hemophilia and Colorblindness 17

X Y X X

1. Colorblindness

a sex-linked recessive trait. (C- normal vision; c- colorblindness)

• In order for a female to be color blind, she would have to have two colorblind genes.

• A male needs to only have one colorblind gene.

Colorblindness - is a sex-linked recessive trait. (C- normal vision; c- colorblindness)

• In order for a female to be color blind, she would have to have this many colorblind genes. _______

• A male needs to have this many _______

• It is more likely to have a colorblind male than a colorblind female because

______________________________________

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2

1

The male only needs one gene

What do you see?

A woman is heterozygous for Normal vision. She marries a man who is colorblind.

What is the predicted colorblindness outcome for their children?

LET … N = normal vision and

n = color blind

Parent genotypes:

XN Xn x XnY

Mom Dad

FIND THE:Genotypic Ratio: Phenotypic Ratio:

X N Xn

Xn

Y

Answer:

LET … N = normal vision and n = color blind

MOM = heterozygous normal vision. XN Xn

DAD = colorblind. XnY

X N Xn

Xn

Y XN Y

Xn XnXN Xn

Xn Y

Genotypic Ratio

1XNXn : 1XnXn : 1 XNY : 1 XnY

Phenotypic Ratio

1 normal female carrier : 1 colorblind female :

1 Normal male : 1 colorblind male 22

X Chromosome Inactivation• Males and females can differ in sex linked traits.

– The expression of genes on the sex chromosomes differs from the expression of autosomal genes.

– Genes located on the sex chromosomes are called sex- linked genes or X-linked genes.

– Males express all of the alleles on both sex chromosomes.– In females one of the two X chromosomes is randomly

turned off by a process called X chromosome inactivation.

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Environment + Genes• Your phenotype is not just dependent on your

genes, but also the environment

• Examples– Sea turtles gender –determined by the temp of

sand– Gender of alligators – determined by temp.– Height of people- influenced by nutrition

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A pedigree is a chart for tracing genes in a family.

• Phenotypes are used to infer genotypes on a pedigree.• Autosomal genes show different patterns on a pedigree than

sex-linked genes.

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