Human Genetics

Human Genetics
Ch 14: Autosomal Dominant, Autosomal Recessive, and Sex-linked Disorders and Pedigrees

Honors Biology
What is the difference between an Autosome and a Sex-chromosome?
Autosomes are the first 22 homologous pairs of human chromosomes that do not influence the sex of an individual.

Sex Chromosomes are the 23rd pair of chromosomes that determine the sex of an individual.
Autosomal Traits
Genes located on Autosomes control Autosomal traits and disorders.

2 Types of Traits:
Autosomal Dominant
Autosomal Recessive
Autosomal Dominant Traits
If dominant allele is present on the autosome, then the individual will express the trait.

A = dominant a = recessive

What would be the genotype of an individual with an autosomal dominant trait?
AA and Aa (Heterozygotes are affected)

What would be the genotype of an individual without the autosomal dominant trait?
aa
Autosomal Recessive Traits
If dominant allele is present on the autosome, then the individual will not express the trait. In order to express the trait, two recessive alleles must be present.

A = dominant a = recessive

What would be the genotype of an individual with an autosomal recessive trait?
aa

What would be the genotype of an individual without the autosomal recessive trait?
AA or Aa
Aa – called a Carrier because they carry the recessive allele and can pass it on to offspring, but they do not express the trait.
Sex-Linked Traits
Sex-linked traits are produced by genes only on the X chromosome.
They can be Dominant or Recessive.

A = dominant a = recessive

What would be the genotypes of a male and female that have a Sex-linked Dominant trait and do not express the trait?
Expresses Trait: Male - XA Y Female - XA XA or XA Xa
No Expression: Male - Xa Y Female - Xa Xa

What would be the genotypes of a male and female that have a Sex-linked Recessive trait and do not express the trait?
Expresses Trait: Male - Xa Y Female - Xa Xa
No Expression: Male - XA Y Female - XA XA or XA Xa
(Carrier)

Most Sex-linked traits are Recessive!
Genetic Counselor Activity
Imagine that you are a Genetic Counselor assigned to family to discuss with them the possibility of their child inheriting a genetic disorder.

You are given the family history and whether or not the disorder is Autosomal Dominant or Autosomal Recessive.

Draw Punnett Squares to determine odds of children inheriting the disease and answer the questions on the worksheet.
How to Construct a Pedigree?
A Pedigree is a visual showing the pattern of inheritance for a trait. (Family tree)

Symbols and Rules:
Male = Female =
Affected = Unaffected = Carrier =
Link parents together with a line and then make a vertical line to connect to offspring.
Autosomal Dominant Pedigree
Draw a Pedigree showing a cross between Heterozygous parents that have 2 boys and 2 girls. (Show all possibilities)
Genotypes of Affected and Unaffected:
AA and Aa = Affected aa = Unaffected

Autosomal Recessive Pedigree
Draw a Pedigree showing a cross between Heterozygous parents that have 2 boys and 2 girls. (Show all possibilities)
Genotypes of Affected and Unaffected:
AA=Unaffected Aa=Carrier, Unaffected aa=Affected

Sex-Linked Recessive Pedigree
Draw a Pedigree showing a cross between a Red eyed Male fruit fly and a Carrier Female fruit fly which have 2 males and 2 females. (Show all possibilities) Red is dominant to white.

Genotypes of Parents:
Male = XR Y Female = XR Xr

Characteristics of Autosomal Dominant, Autosomal Recessive, and Sex-linked Recessive Traits
In groups, analyze your notes on each type of disorder and examine the pedigrees.

Come up with rules/characteristics for each type of Trait.
Autosomal Dominant Traits
Heterozygotes are affected
Affected children usually have affected parents.
Two affected parents can produce an unaffected child. (Aa x Aa)
Two unaffected parents will not produce affected children. (aa x aa)
Both males and females are affected with equal frequency.
Pedigrees show no Carriers.
Autosomal Recessive Traits
Heterozygotes are Carriers with a normal phenotype.
Most affected children have normal parents. (Aa x Aa)
Two affected parents will always produce an affected child. (aa x aa)
Two unaffected parents will not produce affected children unless both are Carriers. (AA x AA, AA x Aa)
Affected individuals with homozygous unaffected mates will have unaffected children. (aa x AA)
Close relatives who reproduce are more likely to have affected children.
Both males and females are affected with equal frequency.
Pedigrees show both male and female carriers.
Sex-Linked Recessive Traits
More males than females are affected.
An affected son can have parents who have the normal phenotype. (XAY x XAXa)
For a daughter to have the trait, her father must also have it. Her mother must have it or be a carrier. (XaY, XaXa, XAXa)
The trait often skips a generation from the grandfather to the grandson.
If a woman has the trait (XaXa), all of her sons will be affected.
Pedigrees show only female carriers but no male carriers.
Examples of Autosomal Dominant Disorders
Dwarfism
Polydactyly and Syndactyly
Hypertension
Hereditary Edema


Chronic Simple Glaucoma – Drainage system for fluid in the eye does not work and pressure builds up, leading to damage of the optic nerve which can result in blindness.
Huntington’s Disease – Nervous system degeneration resulting in certain and early death. Onset in middle age.
Neurofibromatosis – Benign tumors in skin or deeper
Familial Hypercholesterolemia – High blood cholesterol and propensity for heart disease
Progeria – Drastic premature aging, rare, die by age 13. Symptoms include limited growth, alopecia, small face and jaw, wrinkled skin, atherosclerosis, and cardiovascular problems but mental development not affected.
Examples of Autosomal Recessive Disorders
Congenital Deafness
Diabetes Mellitus
Sickle Cell anemia
Albinism
Phenylketoneuria (PKU) – Inability to break down the amino acid phenylalanine. Requires elimination of this amino acid from the diet or results in serious mental retardation.


Galactosemia – enlarged liver, kidney failure, brain and eye damage because can’t digest milk sugar
Cystic Fibrosis – affects mucus and sweat glands, thick mucus in lungs and digestive tract that interferes with gas exchange, lethal.
Tay Sachs Disease – Nervous system destruction due to lack of enzyme needed to break down lipids necessary for normal brain function. Early onset and common in Ashkenazi Jews; results in blindness, seizures, paralysis, and early death.
Examples of Sex-Linked Recessive Disorders
Red/Green Colorblindness – Difficulty perceiving differences between colors (red or green, blue or yellow).
Hemophilia – Absence of one or more proteins necessary for normal blood clotting.
Deafness
Cataracts – opacity in the lens that can lead to blindness
Night blindness – (Nyctalopia) rods do not work so that can not see in the dark
Glaucoma – pressure in the eye that can lead to optic nerve damage and blindness
Duchenne Muscular Dystrophy – progressive weakness and degeneration of skeletal muscles that control movement due to absence of dystrophin (protein that maintains muscle integrity). Mainly in boys, onset 3-5 yrs, by 12 years can’t walk, and later needs respirator.
Karyotype Activity
Objective: To learn how to construct a Karyotype and discover different genetic diseases from a karyotype.

Procedure:
Work in groups of 2-3.
Construct a karyotype from one smear
Use “Information on Chromosome Disorders” to identify the type of mutation.
Answer questions on the handout.
What are Chromosomal Mutations?
Damage to chromosomes due to physical or chemical disturbances or errors during meiosis.
Two Types of Chromosome Mutations:
Chromosome Structure
Chromosome Number
Problems with Chromosome Structure:
Deletion – during cell division, especially meiosis, a piece of the chromosome breaks off, may be an end piece or a middle piece (when two breaks in a chromosome occur).
Inversion – a segment of the chromosome is turned 180°, same gene but opposite position
Translocation – movement of a chromosome segment from one chromosome to a non-homologous chromosome
Duplication – a doubling of a chromosome segment because of attaching a broken piec form a homologous chromosome, or by unequal crossing over.
Problems with Chromosome Number

Monosomy – only one of a particular type of chromosome (2n -1)
Trisomy – having three of a particular type of chromosome (2n + 1)
Polyploidy – having more than two sets of chromosomes; triploids (3n = 3 of each type of chromosome), tetraploids (4n = 4 of each type of chromosome).

How do you think Chromosomal Mutations with differing number of chromosomes develops?
Monosomy and Trisomy due to Nondisjunction – members of homologous chromosomes do not move apart in Meiosis I or sister chromatids do not separate during Meiosis II leaves one cell with too few chromosomes and one cell with too many.
Triploids develop from the fertilization of an abnormal diploid egg, produced from the nondisjunction of all chromosomes. Tetraploids develop from the failure of a 2n zygote to divide after replicating its chromosomes, subsequent mitosis would produce 4n embryo.

Polyploidy is common in the plant kingdom, spontaneous origin of polyploid individuals plays important role in evolution of plants. In the animal kingdom, natural occurrence of polyploids is extremely rare. In general, polyploids are more nearly normal in appearance than having monosomy or trisomy, which is more disruptive to have one extra chromosome in a pair.