Biology 3 Marks Question

Pedigree analysis is a record of occurrence of a trait in several generations of a family. It is based on the fact that certain characteristic features are heritable in a family, for example, eye colour, skin colour, hair form and colour, and other facial characteristics. Along with these features, there are other genetic disorders such as Mendelian disorders that are inherited in a family, generation after generation. Hence, by using pedigree analysis for the study of specific traits or disorders, generation after generation, it is possible to trace the pattern of inheritance. In this analysis, the inheritance of a trait is represented as a tree, called family tree. Genetic counselors use pedigree chart for analysis of various traits and diseases in a family and predict their inheritance patterns. It is useful in preventing hemophilia, sickle cell anemia, and other genetic disorders in the future generations.

A cross between tall plant with yellow seeds (TtYy) & tall plant with green seed (Ttyy) is given below. Phenotype ratio:

1. Tall and green = 3/8 or 37.5%
2. Dwarf and green = 1/8 or 12.5%

Mendel selected pea plants to carry out his study on the inheritance of characters from parents to offspring.He selected a pea plant because of the following features:

• Peas have many visible contrasting characters such as tall/ dwarf plants, round/ wrinkled seeds, green/ yellow pod, purple/ white flowers, etc.
• Peas have bisexual flowers and therefore undergo self pollination easily.
• Thus, pea plants produce offsprings with same traits generation after generation.
• In pea plants, cross pollination can be easily achieved by emasculation in which the stamen of the flower is removed without affecting the pistil.
• Pea plants have a short life span and produce many seeds in one generation.

When an individual is crossed with the homozygous recessive parent It is called test cross.
Test cross helps in establishing hetero/ homozygosity of dominant trait.

Phenotypic ratio	=	Tall	:	dwarf
		3	:	tt
Genotyopic ratio	=	TT	:	TY	:	tt
	=	1	:	2	:	1

A Punnet square used to understand a typical monohybird croos between tall and Dwarf plant.

1. Short statured/small round head/furrowed tongue/partially open mouth/palm is broad/physical development retarded/psychomotor development retarded/mental development retarded.
2. Both/male and female.
3. Klinefelter's syndrome.
4. Male.
5. Sterile ovaries/rudimentary ovaries, lack of secondary sexual characters.
6. Female.

• A cross to analyse whether genotype of dominant individual is homozygous or heterozygous.
• On crossing with a recessive parent, if 50% of progeny have dominant trait and 50% have recessive trait then the plant is said to be heterozygous.
• The above value points can be considered with the help of a test cross.

First Cross (Case I)
In the first cross between the tall plant and the dwarf plant, equal number of tall and dwarf plants are produced. This is because the tall plant is heterozygous dominant (Tt) and dwarf plant is homozygous recessive (tt). The progenies will be 50% tall and 50% dwarf.
Second Cross (Case II)
In the second cross between the tall plant and the dwarf plant, all the offsprings will be tall. This is because the tall plant is homozygous dominant (TT) and dwarf plant is homozygous recessive (tt).
The two monohybrid crosses can be represented as follows:

1. Control crosses are not possible in case of humans beings.
2. Analysis of traits in several generations of a family/To trace pattern of inheritance/Whether the trait is dominant or recessive/sex-linked or not.

Haemophilia	Thalassemia
Single protein involved in the clotting of blood is affected.	Defects in the synthesis of globin leading to formation of abnormal haemeoglobin.
Sex linked recessive disorder.	Autosomal recessive disorder.
Blood does not clot.	Results in anaemia.

Mendelian disorder = 1

The sex-linked recessive disease, which shows to transmission from unaffected carrier female to some of the male progeny a single protein that is a part of the cascade of proteins involved in the clotting of blood is affected. An affected individual a simple cut will result in non-stop bleeding. The heterozygous female (carrier) for haemophilia may transmit the disease to sons. The possibility of a female becoming a haemophilic is extremely rare because mother of such a female has to be at least carrier and the father should be haemophilic (unviable in the later stage of life). The family pedigree of Queen Victoria shows a number of haemophilic descendants as she was carrier of the disease.

1. In grasshopper males have one X only (XO type), in Drosophila males have one X and one Y (XY type) - Males in both produce 2 different kinds of gametes so heterogametic.
2. In birds female has ZW, produce two kinds of gametes and so heterogametic.

Possibility of the daughter being haemophilic.

1. Recessive.
2. Autosomal.
3. Parents - Aa and Aa.

Third child - aa.

Fourth child - Aa.

Multiple allelism: Generally in an individual/population, only two alleles of a trait govern the character, but in case of ABO blood group, three alleles IA, IB and i are found to govern blood group in human population.Co-dominance: Allele IA and IB when present in an individual, both being dominant express their own types of sugars/traits.

Gain or loss of a chromosome. (due to nondisjunction)

1. Down Syndrome- Additional copy of 21st chromosome/trisomy of 21.
2. Klinefelter’s Syndrome- presence of an additional copy of X chromosome leading to XXY Turner’s Syndrome- absence of one of the X chromosome i.e. 45 with XO.

Short-statured, small round head, furrowed tongue, partially open mouth, broad palm with characteristic palm crease, physical psychomotor & mental development retarded, big and wrinkled tongue, broad flat face, flat back of head, many 'loops' on finger tips.

Test cross.
Both crosses.
Both interpretations.

1.  

Construction of pedigree chart

2. Sex - linked recessive inheritance pattern.

• The communication was not easy in those days, and his work could not be widely publicised.
• His concept of genes as stable and discrete units that controlled the expression of traits and of the pair of alleles which did not ‘blend’ with each other was not accepted by contemporaries as an explanation for the apparently continuous variation seen in nature.
• Mendel’s approach of using mathematics to explain biological phenomena was totally new and unacceptable to many of the biologists of his time.
• Though Mendel’s work suggested that factors (genes) were discrete units, he could not provide any physical proof for the existence of factors and what they were made of.

Sickle cell anaemia/Phenylketonuria/Thalassemia/O Blood group/Non - rolling of tongue/Fused or attached earlobes/Inability to taste PTC (phenylthiocarbamide).
AlternateAnswer

Dominance: one allele expresses itself in the hybrid heterozygous condition, other is suppressed.
Co dominance: both the alleles of a gene express in a heterozygous hybrid containing two dominant alleles.
Incomplete dominance: Neither of the two alleles of a gene is completely dominant over the other in heterozygous condition, the hybrid is intermediate.

1. Linkage, genes on the same chromosome were either closely associated or far apart.
2. Higher percentage of parental combination and fewer percentage of recombinants are observed when two genes are located very close/tightly linked on the same chromosome.
3. Higher percentage of recombinants and fewer percentage of parental combinations are observed when two genes are located far aparta/loosely linked on the same chromosome.

Sex-linked recessive disease, present on X chromosome, Mother should be carrier and father should be haemophilic for a daughter to be affected, The possibility of a female becoming a haemophilic in extremely rate because mother of a such female has to be at least carrier and the father should be haemophilic (unavailable in the later stages of life). Show criss-cross inheritance from mother to son and from haemophilic father to daughter.

Gregor Mendel: Through his work on pea plants, discovered the fundamental laws of inheritance. He deduced that genes come in pairs and are inherited as distinct units, one from each parent. Mendel tracked the segregation of parental genes and their appearance in the offspring as dominant or recessive traits. He recognized the mathematical patterns of inheritance from one generation to the next.
Mendel did not investigate how characteristics are sorted and combined on cellular level. Sutton and Boveri, working independently, suggested that chromosomes could be shown to bear the materials of heredity.
They noted that the behaviour of chromosomes was parallel to the behaviour of the gene and used chromosomes movement to explain Mendel's laws.
Sutton and Boveri: Argued that the pairing and separation of pair of chromosomes would lead to segregation of a pair of factors they carry. Sutton united the knowledge of chromosomal segregation with Mendelian principle and called it "chromosomal theory of inheritance".

1. ‘A’ - meiosis

‘B’ - Mitosis.

2. ‘C’ - Parthenogenesis.

1. 
2.  

• The gene for eye colour is sex–linked and is present on X chromosome.
• The character passes into the male from female and the male passes it to the female in the next generation. Male has only one X–chromosome and one Y–chromosome with no corresponding allele.

Law of dominance and law of segregation are the two laws, Mendel derived from his monohybrid crosses.

1. Law of dominance explains that in a dissimilar pair of factors of a trait, one of them expresses itself (dominant) and the other remains hidden (recessive).

• It also explains the expression of both the traits in the F, progeny, in a ratio of 3 : 1.

2. Law of segregation states that the members of an allelic pair that remained together in the parent/ hybrid, segregate during gamete formation and enter different gametes.

• A homozygous parent produces all gametes that are similar while the heterozygous one produces two kinds of gametes, each having one allele for a trait.

The three basic facts are as follows:

1. Characters are controlled by discrete units, called factors.
2. The factors occur in pairs.
3. In a dissimilar pair of factors (heterozygous), one member of the dominates (dominant) and the other is recessive.

Mendel's work remained unrecognised for the following reasons:

1. His work could not be widely publicised as communication was not easy.
2. His concept of 'factors' as stable and discrete units that controlled the expression of traits and that the pair of alleles which did not blend with each other, were not accepted by his contemporaries as the explanation of variation.
3. Mendel's approach of using mathematical logic to explain the biological phenomena was new and unacceptable to many biologists.
4. Though Mendel's work suggested that factors were discrete units, he could not provide any physical proof for the existence of factors or prove what they are made of.

From the concept of sex determination in humans, it is clear that Y chromosome which is present in males comes from the father and not from the mother. Hence, it is the man who should be blamed for not having a male child rather than the woman. But probability of an X or Y chromosome ending up in the zygote is equal, i.e. 50 : 50. So, it is purely a chance that a girl or a boy is borne and no one should be blamed or rewarded for that. The society should learn to respect the girl child as well.

Artificial selection is the selective breeding of plants and animals to include beneficial traits in them. This is also called selective breeding. While some selective breeding can be purely artificial, many others are natural breeding done in a controlled environment. From the point of ethics, it may be wrong to go for artificial selection. But if we follow the law of natural selection and survival of the fittest, then it can be safely assumed the even in case of artificial selection only those varieties are going to survive which are fit to survive. Hence, it can be inferred that artificial selection is not going to affect the process of natural selection.

Turner’s syndrome:
Cause: Absence of one of the X chromosomes, resulting in the karyotype 44+XO.
Symptoms:

1. Sterile female with rudimentary ovaries.
2. Shield–shaped thorax.
3. Webbed neck.
4. Poor development of breasts.
5. Short stature, small uterus, puffy fingers.

1. It is a dominant trait.
2. It is autosomal
3. Genotypes of parents in generation I:

Female aa; male Aa.

Genotype of third child in generation II: Aa.

Genotype of first grandchild in generation III: Aa.

1. Normal human, because glutamic acid is not replaced by valine at 6th position.
2. The normal person has circular and biconcave RBCs while sufferer has sickle shaped RBCs and suffering from sickle cell anemia.
3. Both males and females because it is a autosomal hereditary disease not sex-chromosomal.

1. Determination of Sex: In human beings the sex of the baby is determined at the time of fertilization.

• The sex chromosome pattern in the human females is XX and that of male is XY. Therefore, all the haploid female gametes (ova) have the sex chromosome X, however, the haploid male gametes have either X or Y.
• After fusion of the male and female gametes, the zygote would carry either XX or XY sex chromosomes depending on whether the sperm carrying X or Y chromosome fertilises the ovum. The zygote carrying XX sex chromosome would be a female baby and XY would be a male baby. Thus, the sex of the child is determined by the father and not by the mother.

2. It is sex chromosomal trisomy called as Klinefelter's syndrome.

The symptoms are:

• Tall stature with feminised physique.
• Enlarged breast.
• Sterility.
• Small testes.