Jan 20, 2016
We discussed earlier in this unit, the usage of karyotype charts
Humans have 23 pairs of chromosomes◦ 1 sex chromosome pair◦ 22 autosome pairs
Recall that males have one X and one Y, while females have two X’s
These split during meiosis, and we can use a Punnett square to determine the possible outcomes for sex of offspring
From this, we can see that the mother can only supply an X chromosome
The sex of the child is solely due to the father
Sperm cells should be produced in equal numbers for X and Y
Recall human blood type has multiple alleles The traditional convention for expressing
dominance and recessiveness no longer works
Alleles are often expressed as superscripts Both A and B types are codominant, and O is
recessive A is IA, B is IB, and O is i The following chart summarizes the
genotypes and phenotypes
Because A and B are codominant, they both show up, but do not blend
The individual will be AB A lack of either results in O
A male heterozygous for blood type A plans to have children with a female who is heterozygous for B. What possible blood types could their offspring have?
IAi
IBi
IA
i
IB i
IAIB IAi
IBi ii
The possible blood types for the offspring would be AB, A, B or O
Additionally, the Rh+ factor is a dominant allele
After identifying the nature of a trait, geneticists often look at family history
By understanding the phenotypes of certain members of a family, they can gather more info about others
This is organized in a pedigree chart
A circle representsa female.
A horizontal line connecting a male and a female represents a marriage.
A shaded circle or square indicates that a person expresses the trait.
A square representsa male. A vertical line and a
bracket connect the parents to their children.
A circle or square that is not shaded indicates that a person does not express the trait.
These only work for traits that are thought to be controlled by genetics alone
Also works best on traits the are due to one gene
Many conditions are due to recessive alleles These will only manifest themselves if a
dominant allele is present An example is cystic fibrosis (CF)
Caused by a recessive allele Sufferers of cystic fibrosis produce a thick,
heavy mucus that clogs their lungs and breathing passageways
The most common allele that causes cystic fibrosis is missing 3 DNA bases.
As a result, the amino acid phenylalanine is missing from the CFTR protein.
Normal CFTR is a chloride ion channel in cell membranes
Abnormal CFTR cannot be transported to the cell membrane
If it does, it will not transport Cl- as easily
Part 1 Part 2
Many other conditions are caused by recessive alleles
Other conditions arise from codominant alleles
In these cases, the heterozygotes have a different phenotype
You saw this with thalassemia in question 12
Sickle cell disease is another example Individuals that are heterozygous for this
usually have normal blood cells, but are resistant to malaria
Finally, some conditions are caused by dominant alleles, although it is uncommon
Disorders Caused by Dominant AllelesDisorder Symptoms
Achondroplasia A type of dwarfism
Huntington’s Disease Loss of neurons, resulting in mental deterioration and loss of muscle control.
Hypercholesterolemia Excess cholesterol in blood