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Mendels Law ofIndependent Assortment Definition: The principles that governheredity were discovered by a monk namedGregor Mendel in the 1860s. One of these principles, now called Mendel’s Law ofIndependent 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 ofparent 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. Mendelnoticed a 9:3:3:1 ratio. About 9 of the F2 plants had green podsand 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 unknowns gametes because the other parent contributes only gametes carrying the recessive allele.
Here’s another example…
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). • 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 = 1/4 Cc x Cc: probability for cc offspring = 1/4Because segregation of each allele pair is an independent event, the rule of multiplication is used to calculate the overallprobability that the offspring will be aabbcc: 1/4 aa x 1/4 bb x 1/4 cc = 1/64
Mendels two laws explain inheritance in terms of discrete factors (genes) which as passed fromgeneration 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.
FA MILY PE DIGRE E A diagram or chart that s hows thepattern of inheritance within a family. A ls o known as a family tree. The chart can inc lude many generation in the s ame family. In all pedigree charts , s quares repres ent males and circles repres ent females .
• The pedigree chart traces a sex-linked trait, the disease hemophilia, through three generations of family members. (The chart is also designed toshow 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 asfrequent as recessive disorders)1. Achondroplasia -dwarfism2. Huntingtons disease -mental deterioration and uncontrollablemovements3. Alzhiemer’s disease -mental retardation usually strikes late in life.4. Hypercholesterolemia -excess cholesterol in blood; heart disease