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
• 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
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