Two copies of each autosomal gene affect

phenotype (physical).

• Mendel studied autosomal gene traits, like hair texture.

Autosome – chromosome with genes not related to sex of organism (body cells)

– Carrier – has an allele for a trait or disease that is not expressed.

– Carrier does not have disease symptoms but can pass it on to offspring.

(dominant)

Dominant allele disorders are rare.

Huntington’s disease is an example of a disease caused by a dominant allele.

. • Genes on sex chromosomes are called sex-linked genes. – Y chromosome - male characteristics .– X chromosome - genes affects many traits.

Males can pass on X or Y

Females only pass on X

Who determines the sex of the offspring?

XX XX

XY XYXY

Sperm

Body Cell

Body Cell

XX

X

X

X

Y

XX

Y

X X

X

1female:1male

Father – he can provide an X or Y chromosome

Egg

• Males have an XY genotype.– All of a male’s

sex-linked genes are expressed.

– Males have no second copies of sex-linked genes

– Y chromosome is much smaller

• Females have an XX genotype.X chromosome inactivation -randomly “turns off”

one X chromosome.

Why are males more likely than females to have genetic disorders?

All sex-linked genes are expressed, even recessive. Females have a backup 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.

XCXC XCXc

XCY XcY

XC

XcXC

YXCXC = normal female

XCXc = female, normal vision (carrier)

XCY = normal vision male

XcY = color blind male

• Some traits are neither totally dominant nor totally recessive.

• Incomplete dominance - when neither gene is totally dominant to the other

- Heterozygous phenotype is intermediate between the two homozygous phenotypes– Example: White flowers and red flowers produce

pink flowers

Codominance• Sickle Cell Anemia

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

RR RR’

RR’ R’R’

R

R’

R’

R

Disease in which the body makes sickle-shaped red blood cells. Sickle-shaped cells don’t move easily through your blood vessels. They’re stiff and sticky and tend to form clumps and get stuck in the blood vessels

.

RR = normal blood

RR’ = some sickle cells, some normal cells

R’R’ = has sickle cell anemia

• Codominant - alleles will both be completely expressed.

– Codominant alleles are neither dominant nor recessive.

– The ABO blood types result from codominant alleles.

• Many genes have more than two alleles.

Example – red and white flower produce a flower with BOTH colors

• Polygenic traits are produced by two or more genes.

Order of dominance: brown > green > blue.

• Epistatic gene - can interfere with the expression of all other genes.

Mice have 5 genes that control fur color.

2 genes for general color

1 for shading

1 for spots

1 epistatic gene for color that overrules all other genes

• Height is an example of a phenotype strongly affected by the environmental factors such as early nutrition and health care.

• The sex of sea turtles depends on both genes and the environment. Warm eggs develop into females

• Phenotype is a combination of genotype and environment.

Gene linkage was explained through fruit flies. • Morgan found that linked traits are on the

same chromosome. Traits can be inherited as a group.

• Chromosomes, not genes, assort independently during meiosis.

Wild type Mutant

• Linked genes are not inherited together every time.

• Chromosomes exchange homologous genes during meiosis.

Linkage maps – map of location of genes on a chromosome. • The closer together two genes are, the more likely

they will be inherited together.• Cross-over frequencies are related to distances

between genes.

• Cross-over frequencies can be converted into map units.– gene A and gene B cross over 6.0 percent of the time

– gene B and gene C cross over 12.5 percent of the time

– gene A and gene C cross over 18.5 percent of the time

Human genetics follows the patterns seen in other organisms. • The basic principles of genetics are the

same in all sexually reproducing organisms.– Inheritance of many human

traits is complex.– Single-gene traits are

important in understandinghuman genetics.

Females can carry sex-linked genetic disorders. • Males (XY) express all of their sex linked genes.• Expression of the disorder depends on which parent carries

the allele and the sex of the child.

X chromosome carries about 1100 genes while the Y carries about 250

Pedigree - 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.

Widow’s peak: W = widow’s peak w = non widow’s peak

• If the phenotype is more common in males, the gene is likely sex-linked.

Colorblindness: M = normal vision m = colorblindness

Several methods help map human chromosomes. • Karyotype - a picture of all chromosomes in a cell.

X Y

• Karyotypes can show changes in chromosomes. – deletion of part of a chromosome or loss of a

chromosome– large changes in chromosomes– extra chromosomes or duplication of part of a

chromosome