principles of inheritance Flashcards
(27 cards)
define mutation
Mutation is a phenomenon which results in alteration of DNA sequences and conseuqently results in changes in the genotype and phenotype of an organism.
define colour blindness
It is a sex-linked recessive disorder due to defect in either red or green cone of the eye resulting in failure to discriminate between red and green colour.
percent of males nd females in which CB occurs
males- 8%, females- 0.4%
haemophilia
- sex-linked recessive disorder
- In this disease, a single protein that is a part of the cascade of proteins involved in the clotting of blood is affected.
- a heterozygous female (carrier) for haemophilia may transmit the disease to her sons.
- Royal disease- queen victoria was a carrier
Reasons for selecting garden pea plant:
- easily available on a large scale
- have a short life cycle
- there are many varieties with distinct characteristics
- they are self pollinated and can cross pollinate easily
reasons for mendel’s success
1.He applied mathematical logic and statistical analysis to interpret the results obtained. Using binomial expression, Mendel had obtained the following results, which tallied with his experimental results- (ax + by)2, where x and y are the two alleles for a character, occurring in a frequency of 50% each, so a and b are the frequency= ½ => 1:2:1
2. He used a large sampling size
3. Connducted experiments for several generations
4. He studied one character at a time then rpoceeded to study inheritance of 2 characters simultaneously.
5. He used available techniques to avoid cross pollination by undesirable pollen grains.
define test cross
a cross in which an unkown genotype of a dominant phenotype is crossed with a double recessive called a test cross.
define reciprocal cross
a cross, with phenotype if each sex reversed as compared to the original, to test the role of parental sex on inheritance patter. for eg: pollen of tall plants transferred to
Law of Dominance
i) Characters are controlled by discrete units called factors
ii) Factors occur in pairs
iii) In a dissimilar pair of factors, one memeber of the pair dominates (dominant) THE OTHER.
why some dominant and some recessive
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.
example of incomplete dominance
Dog flower/ Snapdragon/ Antirrhinum sp.
In a cross between true-breeding red flowered(R) and true-breeding white flowered plants(r), the F1 was pink flowered. Phenotypic ratio turned out to be 1:2:1 because R was not completely dominant over r.
define inclompete domincance
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.
define co dominance
the phenomenon in which the two dissimilar factors for same character are equally expressed, giving rise to a new phenotype which resembles both parents.
multiple allelism
phenomenon in which a character is controlled by more than two alleles, evident in a population is called multiple allelism.
example of codominance
In blood group AB. IA and IB both express their own types of sugars.
pleiotropy
phenomenon in which a single gene can exhibit multiple phenotypic expression.
examples of pleiotropy
-
PLEIOTROPY IN PEA PLANTS:
* Same alleles control both starch grain size and seed shape.
* Starch synthesis has 2 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 seeds, BB seeds are round and bb seeds are wrinkled. Heterozygotes produce round seeds, and so B seems to be the dominant allele.
* However, the starch grains produced are of intermediate size in Bb seeds. -
PLEIOTROPY IN HUMANS: PHENYLKETONURIA
* the disease is caused by a mutation of the gene that codes for the enzyme phenylalaninehydroxylase
* an affected individual lacks this enzyme which converts the amino acid phenylalanine to tyrosine which is needed for melanin synthesis.
* As a result, this conversion is reduced or ceases entirely due to which, phenylalanine gets accumulated and converted to phenylpyruvic acid and other derivatives. Accumulation of these in the brain results in mental retardation and absence of tyrosine results in reduction in hair and skin pigmentation.
* These are also excreted through urine due to their poor absorption by kidneys.
state law of independent assortment
when 2 pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters.
different types of sex determination
- Male heterogamety
(i) either with or without X chromosome for eg: grasshopper, in which, males have only one X chromosome (XO) and females have pair (XX)
(ii) some gametes with x chromosome or y chromosome for eg: humans, males are XY and females are XX - Female Heterogamety
* female gametes have either Z or W and males only have Z for eg: birds- female is ZW and male is ZZ - No. of sets of chromosomes
* in bees, the female is diploid and male is halpoid (haplodiploid sex determination system). the female is formed from fertilization of ovum w sperm and male is development of unfertilised ovum by parthenogenesis.
* males dont have father, hence cannot have sons, can only help with fertilising ovum to make females which can produce sons hence males can have grandsons.
sickle cell anemia
- autosome linked recessive trait
- controlled by single pair of alleles Hb^A and Hb^S
- out of 3 possible genotypes, homozygous HbSHbS show the diseased phenotype, the rest are carriers since there is 50% probability of transmission of mutant gene to the progeny.
- caused by substituition of glutamic acid by valine on the sixth position of the beta globin chain of the haemoglobin. This is due to the single base substituion at the sixth codon of the beta globin chain from the GAG to GUG.
- the mutant haemoglobin molecule undergoes polymerisation under low oxygen tension causing the change in shape of RBC from biconcave disc to elongated sickle like structure.
Thalassemia
- autosome-linked recessive blood disease when unaffected carriers give offspring.
- defect caused either due to mutation or deletion which ultimately results in reduced synthesis of one of the globin chains that make up haemoglobin.
- this causes formation of abnormal haemoglobin molecules resulting in anemia.
types of thalassemia
- Alpha thalassemia
* production of alpha globin chain is affected.
* controlled by 2 closely linked genes HBA1 and HBA2 on chromosome 16 of each parent.
* mutation or deletion of one or more of the four genes-more genes affected, lesser alpha globin molecules produced. - Beta Thalasemia
* production of beta globin chain is affected
* controlled by a single gene HBB on chromosome 11 of each parent.
* mutation of 1 or both of the genes.
how are chromosomal disorders caused?
caused due to absence or excess or abnormal arrangement of one or more chromosomes.
define aneuploidy and polyploidy
- Aneuploidy- failure of segregation of chromatids during cell division cycle results in gain or loss of a chromosome (s) i.e., nondisjunction of chromosomes during anaphase 1 of meiosis 1.
- Polyploidy- failure of cytokinesis after telophase stage of cell cycle division results in a nincrease in a whole set of chromosomes in an organism.
