Mendel's Law of Independent Assortment

Definition: The principles that govern heredity were discovered by a monk named Gregor Mendel in the 1860's. One of these principles, now called Mendel’s Law of Independent Assortment, states that allele pairs separate independently during the formation of gametes. This means

that traits are transmitted to offspring independently of one

another.

Mendel performed dihybrid crosses, mating of parent plants that differ in

two traits in plants that were true breeding for two traits. • For example, a plant that had green pod color and yellow seed color was cross-

pollinated with a plant that had yellow pod color and green seeds. In this cross, the traits for green pod color (GG) and yellow seed color (YY) are dominant. Yellow pod color (gg) and green seed color (yy) are recessive. The resulting offspring or

F1 generation were all heterozygous for green pod color and yellow seeds (GgYy).

Mendel then allowed all of the F1 plants to self-pollinate. He

referred to these offspring as the F2 generation. Mendel

noticed a 9:3:3:1 ratio. About 9 of the F2 plants had green

pods and yellow seeds, 3 had green

pods and green seeds, 3 had yellow

pods and yellow seeds and 1 had a

yellow pod and green seeds.

A test cross is a breeding or a mating between an individual of dominant phenotype, who could be either homozygous dominant (SS) or heterozygous (Ss), with

an individual that MUST be homozygous recessive

(ss). • These Punnett squares show

the two different possibilities. Look them over carefully and convince yourself that, in a test cross, a homozygous individual will produce offspring with only the dominant phenotype, but a heterozygous individual will produce offspring with both phenotypes (in equal numbers).

Notice that the offspring will reflect that ratio of the unknown's gametes because the other parent contributes only gametes carrying the recessive allele.

Here’s another example…

Because segregation of each allele pair is an independent event, the rule of multiplication is used to calculate the overall

probability that the offspring will be aabbcc: 1/4 aa x 1/4 bb x 1/4 cc = 1/64

• A trihybrid cross between two organisms with the genotypes AaBbCc and AaBbCc will result in a 1/64 probability of producing an offspring with the genotype aabbcc. Aa x Aa: probability for aa offspring = 1/4

Bb x Bb: probability for bb offspring = 1/4Cc x Cc: probability for cc offspring = 1/4

The rules of probability applied to segregation and independent assortment can solve complex

genetics problems. For example, Mendel crossed pea varieties that differed in three traits (trihybrid

crosses).

Mendel's two laws explain inheritance in terms of discrete factors (genes) which as passed from

generation to generation according to simple rules of chance.

• These principles apply to all sexually reproducing organisms for simple patterns of inheritance.

• Experiments with many organisms indicate that more complicated patterns of inheritance exist.

• The more complicated patterns of inheritance include situations where one allele is not completely dominant over another allele, there are more than two alleles for a trait, or the genotype does not always dictate the phenotype in a rigid manner.

FAMILY PEDIGREE A diagram or chart

that shows the pattern of

inheritance within a family. Also

known as a family tree. The chart

can include many generation in the same family. In all pedigree charts,

squares represent males and circles

represent females.

• The pedigree chart traces a sex-linked trait, the disease hemophilia, through three

generations of family members. (The chart is also designed to show the possible

combination of genes contributed from a given set of parents.) The gene for

hemophilia is linked to the X chromosome but is most likely to be expressed in males. If

a boy receives a copy of the hemophilia-X chromosome from his mother, he will certainly have the disease. A girl who receives a copy of the hemophilia-X

chromosome from her mother will not necessarily have the disease; she will,

however, be a carrier. In that chart, a circle divided in half indicates that the individual is

a carrier for the trait.

Many Inherited Human disorders are controlled by single gene

• Recessive disorders

1. Cystic fibrosis– most common lethal genetic disorder in US (4% of whites are

carriers)

– due to defective chloride channels causing abnormal concentration of extracellular chloride leading to mucous buildup from mucosal epithelium.

2. Tay Sachs disease– disfunctional enzyme which fails to break down certain lipids

in lysosomes.

3. Sickle-cell anemia– single amino acid replacement in haemoglobin molecule.

Dominant inherited disorders (not as frequent as recessive disorders)

1. Achondroplasia -dwarfism

2. Huntingtons disease-mental deterioration and

uncontrollable movements3. Alzhiemer’s disease

-mental retardation usually strikes late in life.4. Hypercholesterolemia

-excess cholesterol in blood; heart disease