chapter 27

Chapter 27Chapter 27

Human GeneticsHuman Genetics

The Role of ChromosomesThe Role of Chromosomes

Chromosome Numbers in Living ThingsChromosome Numbers in Living Things

1.1. Each human sperm or egg has 23 Each human sperm or egg has 23 chromosomes.chromosomes.

2.2. Each human body cell has 23 pairs Each human body cell has 23 pairs of chromosomes.of chromosomes.

3.3. Different organisms have different Different organisms have different numbers of chromosomes.numbers of chromosomes.

Human Cells

Sex Cells

Body Cells

23 Chromosomes

Chromosomes not paired

46 Chromosomes

Chromosomes arranged in 23 pairs

Copyright © by Holt, Rinehart and Winston. All rights reserved.

ResourcesChapter menu

Section 1 The Role of ChromosomesChapter 27

Detecting Genetic Disease

• Genetic screening examines a person’s genetic makeup and potential risks of passing disorders to offspring.

• Amniocentesis is a way of looking at the chromosomes of a fetus.




• Doctors can also take a small piece of the placenta to obtain a sample of chromosomes.

• The chromosomes are counted and studied to see whether parts are missing.

• Karyotypes are charts where chromosomes are organized in pairs.



Chapter 27

Sex – A Genetic Trait

• Sex Chromosomes and Autosomes– Sex chromosomes contain genes that determine

an organism’s sex (gender).

– The remaining chromosomes that are not directly involved in determining the sex of an individual are called autosomes.

Section 1 The Role of Chromosomes



Chapter 27

Sex – A Genetic Trait, continued

• Sex Determination

– In mammals, an individual carrying two X chromosomes is female.

– An individual carrying an X and a Y chromosome is male.




Chapter 27

Karyotypes: Male and Female





What are your chances of having male or female offspring?

XX XX

XY XY

Eggs

Sperm

Body Cell

Body Cell

XX

XY

X

X

X

Y

X

Y

X X

1female:1male




Male or Female?

Who determines the sex of the offspring?

• The Father, because he donates the Y chromosome.



Section 2 Human TraitsChapter 27

Survey on Human Traits

Attached earlobes is aRecessive trait.

Detached earlobes is aDominant trait.




Survey on Human traits

Being able to roll yourTongue is a dominantTrait.

Not being able to roll yourTongue is a recessive trait.





DominantTraits

FreeEarlobes

DimplesCurlyHair





RecessiveTraits

AttachedEarlobes

NoDimples

StraightHair




Incomplete Dominance

Some traits are neither totally dominant nor totally recessive.

Incomplete dominance is when neither gene is totally dominant to the other.





Snapdragons





• Sickle Cell Anemia

• R = Round blood cells• R’ = Sickle Cells

RR RR’

RR’ R’R’

R

R’

R’

R




Blood Types in Humans

• There are four blood types in humans – A, B, AB and O.

• A and B are dominant to O.



Chapter 27

Genes on the X Chromosome

• Sex-Linked Genes and Traits– Genes found on the X chromosome are X-linked

genes. – A sex-linked trait is a trait whose gene is located

on a sex chromosome.– Because males have only one X chromosome, a

male who carries a recessive gene on the X chromosome will exhibit the sex-linked trait.

Section 2 Human Traits



Chapter 27

Genes on the X Chromosome

• Color blindness is a problem in which red or green look like shades of gray or other colors.

• The gene is carried on the X chromosome and is a recessive trait.

Section 2 Human Traits

XC XC XCXc

XCY XcY

XC

XcXC

Y

chapter 27

Documents

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human traitschapter

human traitscopyright

resourceschapter menuchapter

pairs of chromosomes

autosomessex chromosomes

remaining chromosomes

sample of chromosomes