Genetics

7th Grade – Mrs. Boguslaw

• Genetics = the study of heredity • During meiosis, gametes receive ½ of their

parent’s chromosomes • During sexual reproduction, two gametes (male

and female) come together to form a new organism

• This organism has one set of DNA from mom and one set of DNA from dad

• 23 chromosomes from mom + 23 chromosomes from dad = full set of 46 chromosomes (23 pairs!)

• Allele = different forms of a gene

Introduction and Background

• The “Father of Genetics” • Austrian monk that studied and

experimented with pea plants • Looked at characteristics like flower color,

plant height, pea color, and pea shape • Developed important laws of genetics • Parent x Parent à F1 Generation à F2

Generation

Gregor Mendel

Three generations of data

1. P Generation (Parents) - Cross two purebred plants

2. F1 Generation (Offspring generation #1) - All plants looked the same - Same traits expressed in all of

the F1 generation

3. F2 Generation (Offspring generation #2) - The missing trait reappeared

What did this look like? 
Example: Pea Plant Height

P Generation

Pure tall x Pure short

F1 Generation

All look the same; 100% tall

F2 Generation

Short trait comes back! 75% tall, 25% short

• Different versions of the same gene • Located on homologous chromosomes • Can be dominant or recessive

– Dominant: ALWAYS expressed – Recessive: can be masked by a dominant allele

Alleles

Homozygous Dominant

Same alleles = same case letters Dominant is capitalized TT, PP, GG, DD, etc.

Heterozygous Different alleles = different case letters One capital, one lowercase Tt, Pp, Gg, Dd, etc.

Homozygous Recessive

Same alleles = same case letters Recessive is lower case tt, pp, gg, dd, etc.

Allele Combinations

Genotype vs Phenotype

Genotype

the GENES the organism has

the specific letters (alleles)

that an organism has

Note the ratio!

Phenotype

the PHYSICAL APPEARANCE of

an organism

the expression of the genes

Note the ratio!

• Diagrams to predict the outcomes of genetic crosses

• Can be done for one trait (monohybrid) or two traits (dihybrid)

• Typical setup: female parent alleles on left side, male parent alleles on top

Punnett Squares

• Monohybrid (one trait) : 4 squares • Dihybrid (two traits) : 16 squares

• http://www.youtube.com/watch?v=prkHKjfUmMs

How to draw a Punnett square

http://www.youtube.com/watch?v=prkHKjfUmMs

• Both alleles are expressed, to some degree

• If heterozygous, BOTH traits are expressed and blended together

Incomplete Dominance

• Hair Type – Curly, Wavy, Straight

• Voice pitch in men – high, medium, low

• Some genetic diseases have some degree of incomplete dominance – Tay-Sachs (autosomal recessive): abnormal enzyme

activity in heterozygous carriers

Incomplete Dominance in People

• Neither allele is recessive – both are expressed fully • Human example: blood type AB • Common other examples:

Codominance

Sex-linked

Lets recall how we determine gender…How do we determine the sex of an

individual? à through sex chromosomes ▪ Sex Chromosomes: X and Y Female: XX Male: XY ▪ Who is responsible for gender

determination in the child? The father because he gives an X or Y to

the gametes. The mother only gives an X to the gamete

▪ Which sex chromosome is bigger? à the X chromosome (y is much

smaller)

What are sex-linked genes?
 à genes found on a sex chromosome

• X-linked genes are genes found on the X chromosome, symbolized by Xr, XR, Y.

• Y-linked genes are found on the Y chromosome, symbolized by X, YR, Yr

• Thomas Morgan experimented with the eye color of fruit flies (Drosophilia) to determine X-linkage

X-linked Genes• X-linked recessive traits will show up

more often in males because they have only 1 X chromosome

• Let’s do a Punnett square for a female carrier of an x-linked gene and a recessive male

• What will the genotypes be? • XRXr and XRY

Here’s the results

• For girls: 0% have the trait

• For boys 50% have it. • Probability is higher

for boys because whatever X they get determines the trait, for girls they have to get 2 recessive X’s.

XR Xr

XR XR XR XR Xr

Y0 XR Y0 Xr Y0

• More than two possibilities for alleles (more than just the dominant/recessive patterns we’ve seen so far!)

• Example: A, B, O blood types – Side note: blood types also show codominance (AB)!

Multiple Alleles

• Two or more genes interact to form a trait • Can be influenced by environment (i.e. Nutrition) • Examples include eye color, height, skin color, etc.

Polygenic Traits

Pedigrees

What is a pedigree chart?

• Pedigree charts show a record of the family of an individual

• They can be used to study the transmission of a hereditary condition

• They are particularly useful when there are large families and a good family record over several generations.

© 2007 Paul Billiet ODWS

http://www.saburchill.com/IBbiology/bio_hp.html

Studying human genetics

• Pedigree charts offer an ethical way of studying human genetics

• A genetic counselor will still use pedigree charts to help determine the distribution of a disease in an affected family

© 2007 Paul Billiet ODWS

http://www.saburchill.com/IBbiology/bio_hp.html

Symbols used in pedigree charts

• Normal male • Affected male Normal female Affected female • Marriage

A marriage with five children, two daughters and three sons. The middle son is affected by the condition.

Eldest child ↔ Youngest child

© 2007 Paul Billiet ODWS

http://www.saburchill.com/IBbiology/bio_hp.html

Organizing the pedigree chart

• A pedigree chart of a family showing 20 individuals

© 2007 Paul Billiet ODWS

http://www.saburchill.com/IBbiology/bio_hp.html

Organizing the pedigree chart– Generations are identified by Roman numerals

I

II

III

IV

© 2007 Paul Billiet ODWS

http://www.saburchill.com/IBbiology/bio_hp.html

Organizing the pedigree chart• Individuals in each generation are identified by Arabic

numerals numbered from the left • Therefore the affected individuals are II3, IV2 and IV3

I

II

III

IV© 2007 Paul Billiet ODWS

http://www.saburchill.com/IBbiology/bio_hp.html

• The inheritance pattern of a trait can often be determined by looking at a

pedigree

• A karyotype is a picture of all the chromosomes in a cell

• Karyotypes can be used to detect genetic problems – we can see if there is an extra chromosome, a missing chromosome, or a damaged or incomplete chromosome

Karyotypes

X Y

Normal Human Karyotype – Gender?

Normal Human Karyotype – Gender?

Can you spot any potential abnormalities?

Cri-du-chat Syndrome