Intro to Genetics

Why do we look the way we do?

Review of DNA and Chromosomes

Our DNA wind up neatly into a CHROMOSOME.

Chromosomes contain alleles. Alleles are specific gene clusters on a chromosome.

Review of DNA and Chromosomes There are 46 individual chromosomes in a

human cell. When a person wants to reproduce, half the chromosomes are passed on. 23 from mom 23 from dad

Homologous pair

Chromosome Pair 1

Chromosome Pair 2

You’ll pass on half to your

offspring!

Inheritance of chromosomes Egg + sperm zygote

egg

sperm

zygote

fertilizationmitosis &

development

meiosis

Inheritance of genes After fertilization, you now have a

chromosome from each parent. Alleles (genes) may have same information Alleles may be different information

eye color(blue or brown?)

eye color(blue or brown?)

Gene trait from mom

Gene trait from dad

Inheritance of genes

Alleles are inherited separately from each parent Let’s say you have one allele for brown eyes &

one allele for blue eyes. The colors are separate & do NOT blend

either have brown or blue, no blend

Some alleles mask others brown eye color masked blue brown is dominant

Genes Dominant Gene – The gene that is

expressed Illustrated by a capitol letter

Recessive Gene – The gene that is hidden Illustrated by a lower case letter

Genes If both parents give you the same

gene, we call it homozygous. Purebred Example – Two blue eye genes (bb)

Genes If parents give you two different genes on

the alleles, we call it heterozygous. Hybrid Example – a brown and a blue (Bb) BUT….which of the two traits will be

expressed?

Punnett Squares

To determine the odds of that traits might be passed on to the offspring, we can do Punnett Square.

To do a Punnett Square, you have to know what alleles the parent has. In our example, we will cross two heterozygous

people. (Bb)

Punnett squaresBb x Bb

male / sperm

fem

ale

/ eg

gs

X

BB

Bb bb

BbB

b

B b

Genetics vs. appearanceThere can be a difference between

how an organism looks & its geneticsappearance or trait = phenotype

brown eyes vs. blue eyesgenetic makeup = genotype

BB, Bb, bb

2 people can have the same appearance but have different genetics: BB vs Bb

Genetics vs. appearance

eye color(brown B)

eye color(brown B)

eye color(blue b)

eye color(brown B)

vs.

BB

B

BBb

B

b

How were these brown eyes made?How were these brown eyes made?

Karyotype

A kayrotype is the number and visual appearance of the chromosomes in the cell nuclei of an organism or species.

By studying karyotypes, you can determine what traits are

inherited.

In other words…

Chromosomes are digitally arranged so that they are matched with their homologue or “partner” chromosome.

Homologue chromosomes are the same size, shape, and carry the same genes, and one is inherited from each parent.

They are numbered according to size.

Sex determination with karyotype This karyotype has 23

exact pairs.

Note that #23 chromosomes are both X. Therefore this is a female.

Normal human male Note that #23

chromosomes are X and Y.

Is this person female or male?

Trisomy 21 Abnormality

shown in karyotype

Note that there are three copies of #21 chromosome.

This person has Down Syndrome.

Photos of Down Syndrome patients from the National Down Syndrome Society

Correlation between mother’s age and Trisomy 21 incidence

Monosomy X Abnormality

shown in karyotype

Note this person only has 1 copy of the X chromosome.

This female has Turner’s syndrome.

Turners Syndrome

XXY Male (Extra X) Abnormality

shown in karyotype

Note this person has 3 sex chromosomes.

This male has Klinefelter's Syndrome.

XXY Male (Extra X)

How are DNA samples obtained for karyotypes?

Amniocentesis: obtaining amniotic fluid which has cells from the fetus

Chorionic villi sampling: removing cells from the chorion with fetal tissue

If there are chromosomal number abnormalities, how do they form? Meiosis: the process of creating sperm or

egg from a diploid cell

If there is a mistake when chromosomes are separating, then the resulting sperm or egg will have too many or too few chromosomes.

The Human Genome Project

An international research effort to determine the DNA sequence of the entire human genome. http://www.youtube.com/watch?v=nhoEvAY0ToM