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Law of Dominance, Practice Problems and FAQs

Law of Dominance, Practice Problems and FAQs

Have you ever seen a person who can tongue roll? If you ever study the pedigree history of a person who is a tongue roller, you will see that every once in a while there must have been members in the family who were non-tongue rollers. That means the tongue-rolling character is more often expressed in the family than the non-tongue-rolling character. Do you know why? It is because tongue-rolling is a dominant character while non-tongue-rolling is a recessive one. But what do dominant and recessive characters mean? How did we get to know which one is dominant and which one is recessive? Is there any law to govern the inheritance of these characters? To answer these questions let us go through this article where we will discuss the Law of Dominance as stated by Gregor Mendel.

Table of contents

  • Law of dominance
  • Importance of law of dominance
  • Mendelian deviations
  • Practice problems
  • FAQs

Mendel’s experiments

Mendel, the father of genetics, had extensively studied the principle of inheritance of characters in garden pea plants, Pisum sativum, and laid down three laws of inheritance. Mendel picked up 34 varieties of pea plants for his experiments and allowed them to self-breed for raising pure varieties. The number was reduced to 22 and finally to seven. All of the seven varieties had at least one character with two contrasting traits as given in the table below. Each trait was confirmed to be true breeding by Mendel by allowing the plant to self-pollinate. A true breeding line is one that shows stable trait inheritance and expression for several generations after having undergone continuous self-pollination for several generations. Any plant which showed any deviation was discarded.

Character

Contrasting traits

Stem height

Tall (T) / Short (t)

Flower colour

Violet/red (R) / White (r)

Flower position

Axial (A) / Terminal (a)

Pod shape

Inflated (I) / Constricted (i)

Pod colour

Green (G) / Yellow (g)

Seed shape

Round (R) / Wrinkled (r)

Seed colour

Yellow (Y) / Green (y)

Monohybrid cross

Mendel proposed the law of dominance based on his observations of the monohybrid crosses performed by him. A monohybrid cross between two organisms shows different variations for a single observable character.

Let us take an example of one of Mendel’s Monohybrid crosses where he crosses a true breeding pea plant with yellow seeds and a true breeding pea plant with green seeds. He used 50% of the flowers of the same plant to function as female plants by emasculating them ( removing anthers) and cross-pollinated them by dusting the stigma with pollen from the other 50% of flowers which he used as male flowers by bagging their stigma.

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Fig: Crossing true-breeding pea plant with yellow seeds and a true-breeding pea plant with green seeds

He collected the seeds produced as a result of this cross and planted them to obtain the first generation of hybrid plants which he called as the first filial generation or the F1 generation. Mendel observed that in the F1 generation, all the offspring gave rise to yellow seeds.


                                                 Fig: F1 generation

Upon performing the same cross for other characters, he found out that the F1 generation always reflected only one of the parental traits whereas the other trait disappeared in the F1 generation.

He then self pollinated the F1 hybrids and called the progeny generation as the second filial generation of F2 generation. Mendel found out that some of the offspring in the second filial generation showed the appearance of the green seed colour. Around 3/4th of the total number of offspring showed the appearance of yellow seed colour whereas 1/4th of them showed the appearance of green seed colour.


                                             Fig: F2 generation

He obtained similar results for monohybrid crosses performed to see the inheritance of other characters too. One of the two parental traits was expressed in the F1 generation whereas both the traits appeared in the F2 generation in the ratio 3:1


                                     Fig: Summary of monohybrid cross

Reciprocal cross

Reciprocal cross is a paired cross, in which traits of male and female parents are switched to observe any differences in the traits of offspring. Mendel performed his experiment with yellow and green pea bearing true breeds.

Say he first chose a yellow pea plant to be a female parent and a green pea plant to be male parent.

He performed the cross and obtained F1 progenies. He found that all the F1 progenies were yellow. When F1 was selfed, he found that F2 progeny had yellow and green pea plants in the ratio 3:1.

Now Mendel switched the parents. This time he chose the green pea plant as the female parent and the yellow pea plant as male parent. He obtained F1 and F2 progenies.

Mendel observed that regardless of which parent contributed to which trait, the progenies he obtained were the same. All F1 generations were yellow and the F2 generation had yellow and green pea plants in the ratio 3:1.


                                                  Fig: Reciprocal cross

Mendel’s propositions

Mendel proposed that each character is controlled by a pair of unit factors (nowadays known as genes) which are stably passed down from the parent to the offspring via the gametes. Such pairs of factors or genes which code for a pair of contrasting traits of a character are nowadays called alleles or allelomorphic pairs.

He decided to give the contrasting traits symbols. He proposed to use the first letter in upper case for the trait expressed at the F1 stage and the same letter in lower case for the other trait. So for yellow seeds the symbol will be Y and for green seeds the symbol will be y. This is because yellow seed colour was expressed in the F1 generation. Thus, Y and y are alleles of the same gene. Thus, the pair of alleles that can influence seed colour in the pea plant can be of three types - YY, Yy and yy. If identical alleles (YY or yy) are present in an individual, then it is considered to be homozygous and if non-identical alleles (Yy) are present then it is considered to be heterozygous. The genetic composition of alleles such as YY, Yy and yy is said to be genotype while the observable characteristic such as (yellow seed colour or green seed colour) are the phenotype.

Mendel observed that the phenotype (yellow seed colour) of the F1 hybrid with heterozygous alleles Yy are exactly similar to that of the true breeding parent with homozygous alleles YY. Thus, he proposed that in a pair of dissimilar factors, one is dominant over the other (as in the F1) and hence is called the dominant factor while the other factor is recessive. In this case Y (for

yellow seeds) is dominant over y (green seed), that is recessive. He observed the same behaviour for all the other trait-pairs that he studied.

According to Mendel’s observations the different dominant and recessive traits in pea plants are:

Character

Dominant traits

Recessive traits

Stem height

Tall (T)

Short (t)

Flower colour

Violet/red (R)

White (r)

Flower position

Axial (A)

Terminal (a)

Pod shape

Inflated (I)

Constricted (i)

Pod colour

Green (G)

Yellow (g)

Seed shape

Round (R)

Wrinkled (r)

Seed colour

Yellow (Y)

Green (y)

Punnett square

An English geneticist, Reginald Crundall Punnett, also wanted to make a more systematic way to represent the crosses that Mendel had performed. He came up with the Punnett square in 1905.

Let us look at what the Punnett square for the monohybrid cross discussed above would look like.


                    Fig: Punnett square for F1 generation of monohybrid cross


                            Fig: Punnett square for F2 generation of monohybrid cross

Mendel’s Law of Dominance

Mendel’s Law of Dominance is the first of the three laws of inheritance stated by Mendel. It states that out of a pair of factors or alleles (alternative forms of a gene) representing different traits of a character only one is dominant while the other one is recessive. The dominant always expresses itself whereas the recessive factor is unable to express itself in the presence of the dominant factor.

The law of dominance helps to explain why only the parental characters are expressed in the F1 generation of a monohybrid cross and why both the characters are expressed in the F2 generation. It also explains the proportion of 3:1 obtained at the F2 generation.

Deviations from Mendel’s law of Dominance

The major limitation of Mendel’s law of Dominance are that -

  • Dominance does not always occur in every pair of contrasting characters.
  • Not all characters are controlled by a single pair of contrasting alleles.
  • Sometimes, in a pair of contrasting alleles neither one is dominant, and both the alleles express themselves resulting in an intermediate phenotype. Thus a blending of traits is seen. This is known as incomplete dominance.
  • Co-dominance of alleles is seen in many cases in which both the alleles express equally and both the phenotypes of the individual alleles are observed.

Practice problems

1. Choose the dominant trait of a pea plant.

a. Dwarf stem height
b. Violet flowers
c. Terminal flowers
d. Constricted pods

Solution: Dominant traits are the ones that are expressed even when a single allele of the gene is present. Of the mentioned traits, only violet flower colour is a dominant trait.

Character

Dominant traits

Recessive traits

Stem height

Tall (T)

Short (t)

Flower colour

Violet/red (R)

White (r)

Flower position

Axial (A)

Terminal (a)

Pod shape

Inflated (I)

Constricted (i)

Pod colour

Green (G)

Yellow (g)

Seed shape

Round (R)

Wrinkled (r)

Seed colour

Yellow (Y)

Green (y)

Thus, the correct option is b.

2. The ratio in which the contrasting traits were expressed in the F2 generation of Mendel’s monohybrid cross is

a. 1:2:1
b. 1:3
c. 2:2
d. 3:1

Solution: In his monohybrid crosses, Mendel crossed two true breeding pea plants with contrasting traits. He found that only one of the two parental traits was expressed in all progeny of the F1 generation. When the F1 hybrid offspring were self pollinated, the progeny obtained from them was considered to be the F2 generation and it showed the expression of both the traits in the ratio 3:1.

Thus, the correct option is d.

3. What is a reciprocal cross?

a. A cross between two individuals having one contrasting pair of characters
b. A cross between two individuals having two contrasting pair of characters
c. A cross between the F1 hybrid and the recessive parent.
d. A paired cross in which traits of male and female parents are switched to observe any differences in the traits of offspring

Solution: Reciprocal cross is a cross involving two groups of individuals where half of each group are allowed to function as male and the other half as female and then the traits of male and female parents are switched to observe any differences in the traits of offspring. Thus, option d is correct.

4. Draw a Punnett square to show the results of a monohybrid cross if a true breeding tall pea plant is crossed with a true breeding dwarf pea plant.

Answer: When a true breeding tall (T) plant is crossed with a true breeding (dwarf plant), the Punnett square for F1 generation can be expressed as -


       Fig: Punnett square for F1 generation of monohybrid cross between tall and dwarf pea plant

When F1 hybrids were self pollinated, the progeny obtained belonged to the F2 generation. The Punnett square for the inheritance of characters in the F2 generation can be expressed as -


         Fig: Punnett square for F2 generation of monohybrid cross between tall and dwarf pea plant

FAQs

1. What are the other laws of inheritance stated by Mendel?
Answer:
Mendel stated three laws of inheritance. Apart from the law of dominance, the other two laws are the law of segregation and the law of independent assortment.

Law of segregation states that the two factors which control a pair of contrasting characters do not get mixed up but remain distinct and segregate from each other during gamete formation, so that each gamete carries only one factor of a character and is always pure.

Law of independent assortment states that factors or alleles of different characters assort or separate independently of one another at the time of gamete formation.

2. Why was Mendel’s work not recognised by his contemporary scientists?
Answer:
Mendel’s work was not taken seriously by his contemporary scientists. The various reasons are -

  • He was a monk and hence not recognised as a scientist
  • His statistical analysis was beyond the comprehension of biologists of his time
  • His results were published in an obscure local journal
  • Not much knowledge regarding chromosomes and cell division was available at the time to understand Mendel's propositions.

3. Why did Mendel choose Pisum sativum for his experiments?
Answer:
Mendel chose Pisum sativum because -

  • The pea plant existed in a number of different varieties.
  • It grew in his monastery
  • Pure varieties were available
  • The plant is small but flowers are large enough to be manually handled.
  • Cultivating the plant was easy
  • It is an annual plant with a growing period of a few months.
  • Each plant produces a large number of seeds
  • The flowers are bisexual
  • The flowers are normally self-pollinated but can also be self-pollinated manually
  • The plant produces fertile hybrids
  • Chances of contamination by foreign pollen is less as pollination occurs in the bud stage.

4. Why does a cross between red and white Snapdragon flowers show the appearance of pink flowers in the F1 progeny?
Answer:
The alleles for red (R) and white colour (r) of the Snapdragon flowers show incomplete dominance and hence in F1 all the progeny have genotype Rr and express an intermediate phenotype of pink colour.

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