Theoretical genetics

Ms. Jackie

Important definitions

Genotype- the alleles possesed by an organism

Phenotype- all the characteristics of an organism.

Dominant allele- an allele which has the same effect on the phenotype whether it is present in the homozoygous or heterozygous state

Recessive allele- an allele which only has an effect on the phenotype when present in the homozygous state

Important definitions

Codominant alleles- alleles which only has an effect on the phenotype when present in heterozygous but a greater effect in homozygous individuals.

Locus- The particular position of a gene on homologous chromosomes.

Homozygous- two identical alleles of a gene Heterozygous- having two different alleles of a gene Carrier- an individual that has a recessive allele of a

gene that does not have an effect on the phenotype Test cross- testing a suspected heterozygote by

crossing with a known homozygous recessive.

Construction of Punnett grids

Way of finding he expected ratio of the offspring, giving certain parental phenotypes

Punnett grids

Example using characteristics used by Mendel He study the size of

pea plants and found that tall is dominant over short

Lets start with two pure breeding (homozygous) plants with contrasting traits (tall and short)

F1 generation will have the dominant phenotype tall but will be heterozygous.

Tt Tt

Tt Tt

Phenotypes Corresponding genotypes

Tall TT or Tt

Short tt

T T

t

t

Punnett grids

IF the F1 generation self fertilized we will obtained F2:

¾ tall

¼ short

3:1

1TT:2Tt:1tt

TT Tt

Tt tt

T t

T

t

Lets practice

IF a tall garden pea is crossed with a dwarf garden pea, the F1 are all tall. Predict the result of self fertilization of the F1, using a Punnett square.

Pedigree chart

Used to show the inheritance of certain traits over several generations of humans

Example Queen Victoria hemophilic condition Males are normal or affected Females normal or carriers Hemophilia allele is homozygous lethal as well as

sex linked since it is carried on the X chromosome Males can be XHY (normal) or XhY (affected) Females can be XHXH (normal) or XHXh (carrier)

Queen Victoria and hemophilia

Normal male

Affected male

Normal female

Affected female

Victoria Albert

Victoria Edward Alice Alfred Helena Louise Arthur Leopold Beatrice

George V

George VIDuke of Windsor

Elizabeth Margaret

3

3

Irene Frederick Alexandra NicholasII

Alexis

Multiple alleles

Some genes have more than two allelesMultiple alleles can be found in blood

groups

ABO blood groups

Based on 4 different phenotypes (A;B;AB and O)

Caused by different combinations of 3 different alleles

Phenotypes Genotypes

A IAIA or IAi

B IBIB or IBi

AB IAIB

0 ii

ABO blood groups

Using a Punnett Grid work out the possible combinations of blood groups of a male with blood group B and a female with blood group

Using a Punnett Grid explain why a female with blood group O and a male with blood group AB cannot have children with either pf the parents´ blood group.

How sex chromosomes determine gender

Gender determinationDecided by sex chromosomesFemales has two X chromosomesMales have one X and one Y chromosome

Genes in X and Y chromosomes

The X chromosome is relatively large

Y chromosome is much smallerSeveral genes are located on the X

chromosomeSex linked examples

Colorblindnesshemophilia

Few genes on the YHairy ears

Define sex linkage and state two examples

Genes carried on the sex chromosomes, most often on the X chromosome.

Examples Color blindness

Certain colors cannot be distinguished, and is most commonly due to an inherited condition. Red/Green color blindness is by far the most common form, about 99%, and causes problems in distinguishing reds and greens. Another color deficiency Blue/Yellow also exists, but is rare and there is no commonly available test for it.

Depending on just which figures you believe, color blindness seems to occur in about 8% - 12% of males of European origin and about one-half of 1% of females. Total color blindness (seeing in only shades of gray) is extremely rare.

There is no treatment for color blindness, nor is it usually the cause of any significant disability.

Define sex linkage and state two examples

Hemophilia Blood condition in which an essential clotting factor

is either partly or completely missing. This causes a person with hemophilia to bleed for

longer than normal. Cuts and grazes are not great problems as a little

pressure and a plaster are usually enough to stop bleeding.

The main problem is internal bleeding into joints, muscles and soft tissues.

Hemophilia is a lifelong inherited genetic condition, which affects females as carriers and males who inherit the condition.

Human females can be homozygous, hetero with respect to sex linked genes

Human males cannot be heterozygous since they only have one copy of one alleleExample

ColorblindnessExisting alleles XB for normal vision

Xb for colorblindness

A female can be XBXB or XBXb XbXb

A male can be XBY XbY

Human females can be homozygous, hetero with respect to sex linked genes

Hemophilia example

Existing alleles XH for normal Xh for hemophilia

A female can be XHXH or XHXh XhXh (not exist)

A male can be XHY XhY

•Homozygous lethal•Female carriers are heterozygous for X linked recessive alleles

•They will not show the disease but can pass it on to the next generation

Explain that female carriers are heterozygous for X-linked recessive alleles

Heterozygous females for sex linked traitsAre said to be carriersThey will not show the diseaseCan pass the disease to the next generation

Calculate and predict the genotypic and phenotypic ratios of monohybrid crosses

Monohybrid crosses Involves the cross

between two individuals to study the inheritance of one gene

Example Character:

Tongue Rolling Gene:

R Alleles:

R, r Genotypes:

RR, Rr, rr

Phenotypes: rollers, non rollers

Cross: Heterozygous X

Heterozygous Rr X Rr Gametes R, r X R, r Punnett square

RR Rr

Rr rr

R r

R

r

Ratio 3:1

Calculate and predict the genotypic and phenotypic ratios of monohybrid crosses

In your notebooks calculate the genotypic and phenotypic ratios of a cross between a heterozygous tongue roller and a recessive individual for tongue rolling

Calculations and codominance

Type pf inheritance in which the two alleles of the gene are equally dominant and so they are both expressed in the heterozygous genotype.

Alleles R red W white RR red WW white RW- pink

Calculations and codominance

The two alleles are equally dominant This kind of inheritance produces more

variation. Example

Phenotypes of parents Red X White Genotypes of parents RR X WW Gametes R and W F1- RW all pink Perform a self cross between the F1 generation

Deduce the genotypes or phenotypes of individuals in pedigree charts

Go to page 434-435.Answer exercises 2 and 4