We discussed earlier in this unit, the usage of karyotype charts Humans have 23 pairs of chromosomes ◦ 1 sex chromosome pair ◦ 22 autosome pairs

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

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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