Mendel and the Gene Idea. 4 Mendel’s law of independent assortment: –Each allele pair segregates independently of other gene pairs during gamete formation

Mendel and the Gene Idea


Mendel’s law of independent assortment:– Each allele pair segregates independently of

other gene pairs during gamete formation.


D om in an ce In com p le te D om in an ce C od om in an ce


Codominance:– Inheritance characterized by full expression of

both alleles in the heterozygote.– Blood type

– AA– BB– AB


Incomplete dominance:– Pattern of inheritance in which the dominate

phenotype is not fully expressed in the heterozygote, resulting in a phenotype intermediate between the homozygote dominate and recessive characteristics.

– Blends


Test Cross: The breeding of an organism with an unknown genotype with a homozygous recessive.


Rule of Multiplication:– The probability that an independent events will

occur simultaneously is the product of their individual probabilities!


What is the probability that a trihybrid cross between two organisms with genotypes AaBbCc and AaBbCc will produce an offspring with a genotype aabbcc?

Aa x Aa : probability of aa = 1/4 Bb x Bb : probability of bb = 1/4 Cc x Cc : probability of cc = 1/4 Therefore 1/4 x 1/4 x 1/4 = 1/64


Rule of Addition:– The probability of an event that can occur in

two or more independent ways is the sum of the separate probabilities of the different ways

Mendel and the Gene Idea What fraction of the offspring from the

following cross of garden peas, would show recessive phenotypes for at least two of the three traits? PpYyRr x Ppyyrr– ppyyRr 1/4 x 1/2 x 1/2 = 1/16– ppYyrr 1/4 x 1/2 x 1/2 = 1/16– Ppyyrr 1/2 x 1/2 x 1/2 = 2/16 (1/8)– PPyyrr 1/4 x 1/2 x 1/2 = 1/16– ppyyrr 1/4 x 1/2 x 1/2 = 1/16– 6/16 or 3/8


Multiple Alleles– some genes may have multiple alleles; that is,

more than just two alternative forms of a gene.

– These multiple alleles occupy a single locus– Blood typing A, B, and O

– IA IB and i


Human disorders– Recessive alleles that cause human disorders

are usually defective versions of normal alleles.– Recessively inherited disorders range from

nonlethal traits (albinism) to lethal diseases (cystic fibrosis)


Cystic fibrosis: most common lethal genetic disease in the US among Caucasians

– Frequency 1 in 2500

– 4% of the Caucasians are carriers

– The dominate allele codes for a membrane protein that controls chloride traffic across the cell membrane. -absent in homozygous recessive

– Disease symptoms result from the accumulation of thickened mucus in the lungs and pancreas.

Mendel and the Gene Idea Tay-Sachs disease: Incidence is about 100x higher

in central European Jews than among Mediterranean Jews

– Frequency 1 in 3600– Brain cells of babies are unable to metabolize

gangliosides - lipid– As lipids increase in the brain, the infant suffers

seizures, blindness, and degeneration of motor skills and mental performance.

– Child dies after a few years


Pleiotropy:– The ability of a single gene to have multiple

phenotypic effects.

– There are many hereditary diseases in which a single defective gene causes complex sets of

symptoms Sickle-cell Anemia


Sickle-cell disease: most common in African Americans 1 in 400.– Single amino acid substitution in hemoglobin– Abnormal hemoglobin molecules tend to link

together and crystallize when blood O2 is lower than normal

– Causes the erythrocyte to form a sickle shape– Sickle cells clog small vessels causing pain,

discomfort and fever.


Sickle Cell– 1 in 10 African Americans are carriers– Codominance - carriers may experience

discomfort during periods of low blood Ox levels - other wise function normally

– Believed to be result of malaria infestation in tropical regions


Sickle cell:– The probability of inheriting the same rare

harmful allele from both parents, is greater if the parents are closely related.


Huntington’s disease– Lethal dominate (late acting)– a degenerative disease of the nervous system– does not show until age 35-40– irreversible to death– found at the tip of chromosome #4


PKU (phenylketonuria):– recessive 1 in 15,000– patient cannot break down the amino acid

phenylalanine– build up to toxic levels causes mental retardation– tx with a special diet to avoid toxic levels

Chi Square

X2 = Σ (O-E)2

E

The alternate hypothesis in this experiment is to validate the use of the prediction!

The null hypothesis in this experiment is to discredit the use of the prediction!

Chi Square

Degree of freedom: number of possible phenotypic categories minus one.

Chi Square

Evaluate the calculated value with the chart!

If your calculated value is less than the chart value -- then you fail to reject the null!

Chi Square

Last, evaluate your p= value

If you fail to reject your null, then this value identifies the percent chance that any difference in your results is due to random chance!

Documents

Mendel and the Gene Idea. 4 Mendel’s law of independent assortment: –Each allele pair segregates independently of other gene pairs during gamete formation