Question

2. Set up and complete genetic analysis (problems) involving a testcross to determine an organism's genotype 3. the use of the rule of multiplication or a Punnett Square to determine probable outcomes 4. and the use of pedigrees to determine how a particular human trait is inherited. 5. Identify situations involving variations on Mendel's laws, including incomplete dominance, codominance, pleiotropy, polygenic inheritance, and epigenetics inheritance 6. Describe how the chromosome theory of inheritance explains linked genes. 7. Explain and describe examples of sex-linked inheritance .

Answer 1

2) TEST CROSS

In case of monohybrid and dihybrid crosses the alleles will be homozygous either dominant or recessive, which makes them easy to understand their genotype. But in case of heterozygous condition, the phenotype will be same as that of homozygous dominant. so a test cross will help us to find out their genotype. By definition test cross is a cross in which the unknown individual is crossed against a homozygous recessive parent.

Lets take a simple example . Consider a plant with red flower, its genotype can either be RR or Rr (since red color is dominant over white here). so to find out the genotype we can cross this plant against a homozygous recessive plant with white flowers (rr). The two possible outcomes are ,

i) RR (unknown) X rr (recessive white) ii) Rr (unknown) X rr (recessive white)

gametes: R X r gametes: R r X r

offspring Rr (100% red flowers) offspring: Rr rr (50% red and 505 white)

so if the offsprings generated after crossing our unknown plant with recessive parent , have only red flowers then the genotype of the unknown is RR . On the other hand if it has both red as well as white flowers, the genotype of the unknown is Rr.

3) PUNNET SQUARE

The concept of punnet square was introduced by Reginald C Punnet, which is a tabular or visual form of mendelian inheritance which helps to determine the probability of possible genotype outcomes of a cross.

For example, if a heretozygous plant with genotype TtGg (tall with green seed) is self crossed, we can predict the genotype outcomes easily with a punnet square. The number of collumns and rows will be same as that of the number of the gametes from paternal and maternal origin. In this case maternal and paternal gametes are TG, Tg, tG, tg (so 4 collumns and 4 rows)

  

	TG	Tg	tG	tg
TG	TTGG	TTGg	TtGG	TtGg
Tg	TTGg	TTgg	TtGg	Ttgg
tG	TtGG	TtGg	ttGG	ttGg
tg	TtGg	Ttgg	ttGg	ttgg

4) PEDIGREE

Pedigrees are used to analyse the pattern of inheritance of traits throughout a family over many generations. By predicting the presence or absence of a trait it will help to know about the chances of getting heriditory diseases and their probabilities. Also helps to know how certain alleles are inherited whether they are dominant, recessive, autosomal or sex- linked. Pedigree charts are prepared using standardised symbols.

6) CHROMOSOME THEORY OF INHERITANCE

Chromosome theory proposed by Sutton and Boveri considers chromosomes as the significant key of inheritance.

Cardinal points of this theory,

i) homologous chromosome pairs migrate together during meiosis

ii) there is random sorting of chromosomes from each homologous pairs during gamete formation

iii) gametes from each parent possess only half of their chromosome content, and contribution from each parent is equal

iv) these gametic chromosomes fuse in offspring and form same chromosome content as their parents.

Linkage causes alleles on same chromosome to be inherited together