4.3 THEORETICAL GENETICS

ASSESSMENT STATEMENTSDefine genotype, phenotype, dominant allele, recessive allele, codominant allele, locus, homozygous, heterozygous, carrier, and test cross

Determine the genotypes and phenotypes of the offspring of a monohybrid cross using a Punnett grid.

State that some genes have more than two alleles (multiple alleles)

Describe ABO blood groups as an example of codominance and multiple alleles

Explain how sex chromosomes control gender by referring to the inheritance of X and Y chromosomes

State that some genes are present on the X chromosome and absent from the shorter Y chromosome in humans

Define sex linkage

Describe the inheritance of color blindness and hemophilia as examples of sex linkage

State that a human female can be homozygous or heterozygous with respect to sex linked genes

Explain that female carrier are heterozygous for X-linked recessive alleles.

Predict the genotypic and phenotypic ratios of offspring of monohybrid crosses involving any of the above patterns of inheritance.

Deduce the genotypes and phenotypes of individuals in pedigree charts.

Genotype- the alleles possessed by an organism; each allele is represented by a letter; chromosomes come in pairs and so alleles come in pairs- a genotype is therefore represented by a pair of letters- such as TT or Tt.

Phenotype- the characteristics of an organism; a characteristic may be an external feature, such as hair color or flower color, or internal like sickle-cell anemia

Dominant allele- an allele that has the same effect on the phenotype when in either the homozygous or heterozygous state; the dominant allele is always given a capital letter such as T

Recessive allele- an allele that only has an effect on the phenotype when in the homozygous state; recessive allele is always given a lower case of the same letter as the dominant allele such as t

Locus- the specific position of a gene on a homologous chromosome; a gene locus is fixed for a species- for example, the insulin gene is always found at the same position on chromosome 11 in humans.

Homozygous- have two identical alleles at a gene locus; the alleles may both be dominant or both recessive- for example TT or tt.

Heterozygous- having two different alleles at a gene locus- for example Tt.

Codominant alleles- pairs of alleles that both affect the phenotype when present in the heterozygous state; these alleles are represented in a different way in genetic; a capital letter is chosen to represent the gene and then other (superscript) letters represent the alleles (for example for human blood types A and B are codominant alleles and are represented as IA and IB) .

Test Cross- testing a dominant phenotype to determine if it is heterozygous or homozygous- for example, crossing either TT or Tt with tt; if there are any offspring with recessive phenotypes, then the parent with the dominant phenotype must be heterozygous (Tt).

Carrier- an individual with one copy of a recessive allele which causes a genetic disease in individuals that are homozygous for this allele.

You must understand and remember these terms!!

PUNNETT SQUARES

Punnett Squares can be used to work out all the possible combinations of alleles that van be presented in the offspring of two parents with known genotypes.

Punnett squares show combinations and also help to deduce the probabilities of each one occurring.

STEPS FOR PUNNETT SQUARES

1. Choose a letter to represent the gene. Choose one that has a distinctly different upper and lower cases. The letter should be based on the dominant allele.

2. Represent the genotype of each parent with a pair of letters. Use a single letter surrounded by a circle to represent the genotype of each gamete.

3. Combine pairs of the letter representing the gametes to give all the possible genotypes of the offspring.

4. From the possible genotypes, workout the possible phenotypes of the offspring.

CODOMINANCE AND MULTIPLE ALLELES

Before when looking at inheritance one of the alleles completely dominates the other, so in a heterozygous genotype the phenotype is determined by the dominant allele

In codominance, both alleles have an affect on the phenotype.

This Punnett Square represented smooth or wrinkled peas is known as a monohybrid cross because it involves just one gene with two alleles: A for round and a for wrinkled.

17TH CENTURY DAWN OF MODERN MEDICINE

There are many cases where genes have more than two alleles.

One example is human blood groups.

The ABO human blood grouping is an example of both codominance and multiple alleles.

There are three alleles IA ,IB , and i

IA and IB are codominant and both are dominant to i.

This results in four different phenotypes or blood groups

Genotype Phenotype or Blood Group

IA IA A

IA i A

IB IB B

IB i B

IA IB AB

ii O

A person’s blood group depends on which combination of alleles they receives. Each person has only two of the three alleles and they are inherited just as though they are alternative alleles of a pair.

Example

Hair shape in humans is a codominance characteristic. Straight hair and curly hair are codominant alleles and the heterozygous has wavy hair. Popeye and Olive Oil both have wavy hair. Deduce the probabilities that their children will have straight hair, curly hair or wavy hair.

HS - straight hair HC – curly hair

Since both Popeye and Olive Oil have wavy hair their genotypes must be HSHC

SEX CHROMOSOMES AND THE CONTROL OF GENDER

Humans have one pair of chromosomes that determine whether we are male or female.

These chromosomes are call the sex chromosomes.

Each person has one pair of sex chromosomes, either XX or XY along with 22 pairs of other pairs known as autosomes.

Females- XX

Males- XY

Sex chromosomes are inherited in the same way as other chromosomes.

1:1 ratio, meaning at fertilization there is always a 50% chance of a girl and 50% chance of a boy.

Sex Chromosomes and Genes

Sex chromosomes not only carry the genes that control gender, the X chromosome also carries genes called sex linked or X-linked genes.

These genes only on the X chromosome which is larger than the Y.

The Y chromosome carriers alleles that are mainly dealing with male structures and functions.

SEX LINKAGE

Sex linkage- the pattern of inheritance that is characteristic for genes located on the X chromosome.

Sex linkage has a significant effect on genotypes.

Females have two X chromosomes, so they have two alleles for each gene and my be homozygous or heterozygous.

In a female, a single recessive allele will be masked by a dominant allele on the other X chromosome.

Males only have one allele on their X chromosome with no corresponding allele on the Y chromosome, so a recessive allele will always be expressed in a male.

A female who is heterozygous for a sex linked recessive characteristic that does not affect her phenotype is a called a carrier.

EXAMPLES OF SEX-LINKED CHARACTERISTICS

Two examples of sex-linked human characteristics are hemophilia and red-green color blindness.

Hemophilia is a condition in which the blood of an affected person does not clot normally.

HEMOPHILIA

Carrier

The genes controlling the production of the blood-clotting protein factor VIII are on the X chromosome.

This is a pedigree chart showing how a sex-linked condition may be inherited.

Females Xh Xh rarely survive and usually results in a miscarriage.

PEDIGREE CHARTS

Pedigree charts are a way of tracing the pattern of inheritance of a genetic condition through a family tree.

Horizontal lines linking male and female in a generation indicate a marriage and the vertical lines indicate their offspring. Offspring are shown in their order of birth.

RED-GREEN COLORED BLINDNESSA person with red-green colored blindness has difficulty telling red and green apart from one another.

Red-green colored blindness in a inherited in a similar way to hemophilia.

A female who is XBXb is a carrier, and a male with just one copy of the recessive allele Xb Y will be color blind.

PRACTICE QUESTION

A woman who is homozygous for normal vision married a man who is red-green color blind. Determine the possible types of vision inherited by their two children, one girl one boy.

XB normal vision Xb color blindness

Step 1- determine the parents genotypes.Step 2- create a Punnett Square Step3- what will the boy’s and girl’s geno/phenotpye

The pedigree chart shows the family history of a recessive human condition called woolly hair.

A filled in symbol indicates that the person has woolly hair. Deduce whether this is a sex linked condition.

_________

___________________

___________________

______________________________________

I

II

III

1 2

1 2 3

1 23

Practice Problems:

Go to: http://www.zerobio.com/drag_gr11/pedigree/pedigree_overview.htm