Unit 3: Genetics

- GENES are segments of DNA that contain

the information for an hereditary trait. They are

located in the chromosomes

- Genes are transmitted from parents to

offspring through the gametes.

- Each gene has alternatives called

ALLELES. In the population there may be

many, but in an organism there are two, one

in each homologous chromosome.

- The position a gene has in a chromosome

is called LOCUS (plural: loci)

What Are Genes?

Individuals that have two identical alleles for a gene are called

HOMOZYGOUS (AA or aa)

If the two alleles are different, they are HETEROZYGOUS or

HYBRID for that trait (Aa)

GENOTYPE is the group of genes of an

individual. For a trait it may be AA, Aa or aa.

PHENOTYPE is the external expression of

the genotype, that is, the observable trait.

Phenotype does not only depend on the

genotype, but also on the environment in

which the individual develops.

Multiple alleles make possible multiple phenotypes in a population

For each gene, there are three possible genotypes. For the

gene that regulates the production of melanine, for example:

Genotypes Phenotypes

MM-------------------------------------- normal (pigmented skin)

Mm-------------------------------------- normal (pigmented skin)

mm--------------------------------- ---- albino

This happens because the allele M is DOMINANT over the

allele m, that is RECESSIVE.

Dominant alleles have the same effects whether they are in

homozygosis (MM) or heterozygosis (Mm).

Recessive alleles are expressed only in homozygosis (mm)

Sometimes there is no complete

dominance of an allele over the other,

but an INCOMPLETE DOMINANCE, or

INTERMEDIATE INHERITANCE. Then

the heterozygous phenotype is

intermediate between the dominant and

the recessive.

Sometimes both the alleles express simultaneously (CODOMINANCE).

An example of this happens with the bloodtypes.

GAMETES are HAPLOID (n), so they only have one chromosome

for each homologous pair. This means they only have one alelle

for each gene.

Genotype of the individual Gametes it produces

AA A (100%)

aa a (100%)

A (50%)

Aa

a (50%)

A heterozygous will produce 50% of the gametes with each

possible allele.

PHENOTYPE normal albino

GENOTYPE Mm mm

GAMETES PRODUCED M m m

FERTILIZATION

GENOTYPES Mm mm

PHENOTYPES normal (50%) albino (50%)

X

P(parents)

F1

(Offspring)

In fertilization, gametes

combine randomly

Mendel's Good Move:Mendel's success in discovering the laws of

inheritance depended on his wise decision to choose

the pea (Pisum sativum) for his experiments. This was

because:

- it has a short life cycle

- it produces many offspring in one cross

- it allows both self and cross-fertilization and it is easy

to manipulate

- it has easily observable traits. He focused on 7, and

studied its transmission with the aid of statistics:

The traits Mendel studied in peas (Pisum sativum):

When crossing two homozygous parents

(or pure lines), the resulting offspring (F1

generation) is uniform.

MENDEL'S FIRST LAW:

UNIFORMITY OF THE FIRST GENERATION

100% phenotype R

MENDEL'S SECOND LAW:

SEGREGATION

During gamete formation each allele of a pair is

separated (=segregated) from the other

member, and will join again with another one

after fertilization.

75% phenotype R25% phenotype r

First Law: Uniformity of F1

Second Law: Segregation

First Law: Uniformity of F1

Second Law: Segregation

What happens with more than one trait?

To study this, Mendel crossed two pure lines (homozygous) for two traits:Seed colour: yellow (A) > green (a)Seed shape: round (B) > wrinkled (b)

P: AABB x aabb

(yellow,round) (green,wrinkled)

F1: AaBb (yellow,round)

F2: yellow,round yellow,wrinkled green,round green,wrinkled

9 : 3 : 3 : 3

MENDEL'S THIRD LAW:

INDEPENDENT ASSORTMENT

Genes for each trait are transmitted

and inherited independently.

Punnett Squares - They are diagrams used to predict the outcome of a breeding or

cross experiment.

- In these diagrams, we represent in a table the possible alleles

produced by the two individuals that are being crossed. We

assume that the probability of inheriting copies of each parental

allele is independent (according to Mendel's Laws).

One trait Two traits

How Is Sex Determined?

Multiple Alleles: Blood Types

Sex-linked (X-linked) recessive traits: Hemophilia, color-blindnessSome traits (diseases like hemophilia or color-blindness are

determined by alleles whose loci are in the X chromosome.

Males will inherit only one allele from their mother (and always

express it), while females will receive two alleles, one from their

mother and other from his mother.

Tracing Hemophilia through Queen Victoria's Pedigree

Chromosomal abnormalities

Down Syndrome: trisomy 21

Abnormalities in sex chromosomes:

Turner syndrome: X_ Klinefelter syndrome: XXY