Principles of Inheritance and Variation 3 Flashcards
what is the law of dominance
(i) Characters are controlled by discrete,solid units called factors.
(ii) Factors occur in pairs.
(iii) In a dissimilar pair of factors one member of the pair dominates
(dominant) the other (recessive).
what does law of dominance explain
The law of dominance is used to explain the expression of only one of
the parental characters in a monohybrid cross in the F1
and the expression
of both in the F2
. It also explains the proportion of 3:1 obtained at the F2
.
why are some alleles recessive while others are dominant
Variations of genes are called alleles. A wild-type allele is one that is more common in a population of a species and is considered a “normal allele,” while uncommon alleles are considered mutations. Recessive alleles often lead to reduced function or loss of function. A gene has information for synthesis of a protein which is responsible for a trait (phenotype). A dominant gene will often produce enough of its related protein to result in its signature trait, even if it has to do all the legwork on its own. So when it’s paired with the recessive allele — which doesn’t signal the body to produce the protein — the physical result is the same as if it were paired with another dominant one. With two recessive alleles, however, the body is only getting a low dose or no dose of the protein, or a non-functional protein.
state law of segregation
When a diploid sexually reproducing organism undergoes gamete formation, the alleles of the genes seperate such that each gamete receives only one allele.
This is also called Law of Purity of gametes because the gametes produced have only 1 allele and hene is said to be pure
When 2 factors of a character come together, they do not mix, blend with each other. They remain as separate entities. During sexual rep, gametes are formed. Factors for a character segregate from each other and enter different gametes. So each gamete has only one type of information in it, for a character, hence it is pure.
What is incomplete dominance
When both alleles are expressed incompletely ,resulting in a new phenotype intermediate b/w 2 parents, giving rise to a new phenotype. The intermediate phenotype does not resemble either parent. This is called incomplete dominance.
The phenomenon in which two dissimilar factors (alleles) for the same character are expressed unequally, giving rise to a phenotype which is intermediate between the parental phenotypes and does not resemble either parent. The phenotypic and genotypic ratios in F2 are same in such case.
give an example for incomplete dominance
The
inheritance of flower colour in the dog flower
(snapdragon or Antirrhinum majus) is a good example
to understand incomplete dominance.Another example is *Mirabilis jalapa. *
In a cross
between true-breeding red-flowered (RR) and truebreeding white-flowered plants (rr), the F1
(Rr) was
pink (Figure 5.6). When the F1
was self-pollinated
the F2
resulted in the following ratio 1 (RR) Red: 2
(Rr) Pink: 1 (rr) White. Here the genotype ratios were
exactly as we would expect in any mendelian
monohybrid cross, but the phenotype ratios had
changed from the 3:1 dominant : recessive ratio.
What happened was that R was not completely
dominant over r and this made it possible to
distinguish Rr as pink from RR (red) and rr (white) .
Describe the genes which control blood groups
ABO blood groups are controlled by
the gene I. The plasma membrane of the red blood cells has sugar polymers
that protrude from its surface and the kind of sugar is controlled by the
gene. The gene (I) has three alleles I
A, IB and i. The alleles IA and IB produce
a slightly different form of the sugar while allele i does not produce any
sugar. Because humans are diploid organisms, each person possesses
any two of the three I gene alleles.
explain which alleles of gene I are dominant and which are recessive
IA and IB are completely dominant over
i, in other words when IA and i are present only IA expresses (because i does not produce any sugar), and when IB and i are present IB expresses.But when IA and IB are present together they both express their own types of sugars: this is because of co-dominance.
what is codominance
When 2 alleles are expressed equally, it gives rise to a new phenotype that is a combination of the parentals and resembles both the parents.
How does blood grouping differ from mendelism
i) 2 alleles are dominant
Therefore there is a new phenotype created such that both parental traits are shown by the offpsring. However, there is no mixing or blending.
ii) The gene is controlled by 3 alleles. However, the diploid organism has only 2 of 3 alleles/
what is multiple allelism
When a gene has more than 2 alleles for the same locus, It is called as multiple allelism.
Eg: ABO blood grouping
Since in
an individual only two alleles can be present, multiple alleles can be found
only when population studies are made.
Give an example of pleiotropy
Occasionally, a single gene product may produce more than one effect.
For example, starch synthesis in pea seeds is controlled by one gene. It
has two alleles (B and b). Starch is synthesised effectively by BB
homozygotes and therefore, large starch grains are produced. In contrast,
bb homozygotes have lesser efficiency in starch synthesis and produce
smaller starch grains.
After maturation of the seeds, BB seeds are round
and the bb seeds are wrinkled. Heterozygotes produce round seeds, and
so B seems to be the dominant allele. But, the starch grains produced are
of intermediate size in Bb seeds.
So if starch grain size is considered as
the phenotype, then from this angle, the alleles show incomplete
dominance.
Gene B controls both seed shape and starch grain.
what is dominance
dominance is not an autonomous feature of a gene or the
product that it has information for. It depends as much on the gene
product and the production of a particular phenotype from this product
as it does on the particular phenotype that we choose to examine, in case
more than one phenotype is influenced by the same gene.
define pleiotropy
When a gene controls more than 1 character, It is said to exhibit pleiotropy.
instances where a single gene can exhibit multiple
phenotypic expression. Such a gene is called a pleiotropic gene.
The
underlying mechanism of pleiotropy in most cases is the effect of a gene
on metabolic pathways which contribute towards different phenotypes.
give an example of pleiotropy
An example of this is the disease phenylketonuria, which occurs in
humans. The disease is caused by mutation in the gene that codes for the
enzyme phenyl alanine hydroxylase (single gene mutation). This manifests
itself through phenotypic expression characterised by mental
retardation and a reduction in hair and skin pigmentation.
Phenyl alanine hydroxylase is an enzyme that is required to convert phenyl alanine into tyrosine. Tyrosine can be further converted into melanin.
In the absence of the enzyme, tyrosine is not formed and instead phenylpyruvic acid is formed instead. The lack of tyrosine leads to a loss of melanine and hence reduced pigmentation. A buildup of phenylpyruvate in the brain causes mental retardation.
There is also excretion of phenylpyruvic acid through urine.
