Transcript Beyond Dominant & Recessive Alleles

Sex-linked Genes & Genetic
Disorders
Unit 5: Genetics
Ch. 11-3 & Parts of Ch. 14
Learning Goals
• 1. Describe where sex-linked traits are
located and and why these traits
generally affect men more than
women. Give an example of a sexlinked trait.
• 2. Describe dominant and recessive
disorders and give an example of each.
• 3. Explain nondisjunction and how it
leads to trisomy and monosomy.
Sex-Linked Genes
X Chromosome
• Some genes are carried
on the sex
chromosomes (X & Y)
– XX = female, XY = male
• Several disorders are
carried on the X
chromosome
Y Chromosome
• Because men have only 1 X, they don’t
have a “backup” X like women
• If the X is bad, men have the
disease
• EX: Colorblindness, hemophilia,
muscular dystrophy
Color-Blindness
– Three genes for color vision are located
on the X chromosome.
– In males, a defective version of any one of
these genes produces colorblindness.
• Sex-linked Crosses
– 1) Must show sex
chromosomes for
each parent &
alleles for each X
chromosome
– 2) Answer must
include male or
female
Predict Gender & Phenotypes
• Colorblindness is a recessive trait
• Geno:
Pheno:
– XCXC Normal Female
– XcXc Colorblind Female
– XCXc Normal / Carrier Female
– XCY Normal Male
– XcY Colorblind Male
Pedigree Charts
• A chart used to examine the
appearance of traits, especially
diseases, over several generations.
Genetic Disorders
• Some genes code for nonfunctional
proteins.
• These cause hereditary diseases.
Dominant Disorders
• If you have the gene, you have the
disease
– May not show up until later in life, after
you’ve had kids and passed it on
– Huntington disease, Dwarfism
Huntington’s Disease
• Results in the early death of some
neurons in the brain. People loose the
ability to control their movements.
• Symptoms usually don’t start until the
20s to early 30s.
• The disease is fatal, usually by the late
30s or early 40s.
Recessive Disorders
• Can remain hidden and skip
generations
• Heterozygous individuals (Rr) are
called “carriers”
– Albinism, cystic fibrosis, Tay-Sachs,
phenylketonuria
• Cystic Fibrosis- Protein does not
fold properly. Causes a build up of
mucus in the lungs
Chromosomal Disorders
• 1) Nondisjunction: chromosomes do
not separate properly during meiosis
• The results are cells with abnormal
numbers of chromosomes.
– Cells end up with extra
chromosomes (47) or fewer
chromosomes (45).
During Meiosis
• Trisomy: If there are three
homologous chromosomes
instead of two
–Down syndrome is
caused by trisomy of
chromosome 21
Where is the mutation?
Where is the mutation?
• Monosomy: the cell is missing
one chromosome
–Turner’s syndrome is a
female with one “X”
chromosome not 2 “X”
chromosomes.
• 2) Alterations in Chromosome
Structure
– Deletion: portion of chromosome is
deleted
• EX: Cri du chat (Cry of the cat)
– Duplication: piece of chromosome is
repeated
• Ex: Fragile X Syndrome
– Translocation: piece of chromosome is
deleted from one homologous pair and
reattached to another homologous pair
Deletion: Cri du chat
Duplication: Fragile X
Translocation
Learning Goals
• 1. Describe where sex-linked traits are
located and why these traits generally
affect men more than women. Give an
example of a sex-linked trait.
• 2. Describe dominant and recessive
disorders and give an example of each.
• 3. Explain nondisjunction and how it
leads to trisomy and monosomy.
Whiteboard Practice
1. Legend & Parents
2. Punnett Square & Answers
• Queen Victoria was a carrier of
hemophilia (h), a recessive, sex-linked
disease. King Edward was normal (H).
–
–
–
–
1. Make a legend
2. Genotypes of parents?
3 & 4. Show the cross.
5. Give probabilities for both sons and
daughters
• Muscular Dystrophy (d), is a recessive,
sex-linked disease. A man who has
muscular dystrophy marries a woman
who is homozygous normal
–
–
–
–
1. Make a legend
2. Genotypes of parents?
3 & 4. Show the cross.
5. Give probabilities for both sons and
daughters