Inheritance of one trait Gregor Mendel Pages

Gregor Mendel (1822–1884), an Austrian monk, studied garden pea plants as means to explain the inheritance of characteristics.

He conducted a series of experiments on plants over an eight-year period (1853–1861). Following his experiments, Mendel published a paper outlining his conclusions. Mendel conducted his research using the common pea plant (Pisum sativum),. These plants were easy to cultivate and had several traits that could be studied

The seven traits that Mendel studied

How did Mendel set up his experiment? The first thing he needed to do was to obtain purebred plants for the trait he wanted to study. A purebred organism is descended from ancestors of a distinct type, or breed. Purebred organisms in a given species or variety all share similar traits.

Mendel’s First Experiment: A Monohybrid Cross parent generation, P generation. Offspring filial generation, F1 generation. monohybrid cross, A dominant trait is a characteristic that is always expressed, A recessive trait is a characteristic that is latent (present but inactive) The trait for tall pea plants was dominant over the trait for short plants. All the pea plants grew tall if they possessed the dominant trait for size — tall.

Mendel conducted this experiment many times using true-breeding plants for each of the seven traits he had chosen to study. He obtained the same results every time: one trait was dominant over the other. Mendel concluded that heredity was definitely not just a blending of traits. He also concluded that when plants with two contrasting traits are crossed, one trait is always dominant over the other. This led him to formulate the principle of dominance: when individuals with contrasting traits are crossed, the offspring will express only the dominant trait. The results …

Law of Segregation The next experiment Mendel conducted involved breeding the F1 generation. He allowed the hybrid tall plants of the F1 generation to self-pollinate. This produced the second filial generation, or F2 generation.

Mendelian ratio Three out of four plants in the F2 generation were tall while one was short. Mendel repeated this experiment many times and examined all seven traits. He obtained the same results time after time. The F2 generation resembled one parent from the P generation 75% of the time and the other parent from the P generation 25% of the time. This ratio of 3 : 1 is known as the Mendelian ratio.

Based on his observations of traits in the F1 and F2 generations, Mendel drew the following conclusions: Each parent in his F1 generation starts with two hereditary “factors.” One factor is dominant and the other is recessive. The factors separate in the parent. Only one factor from each parent is contributed to the offspring. Each offspring inherits one factor from each parent. If the dominant factor is present it will be expressed even if the recessive factor is also present. The recessive factor will be expressed if only recessive factors are present.

The law of segregation The law of segregation states that inherited traits are determined by pairs of “factors.” These factors segregate (separate) in the gametes, with one in each gamete.

Genes & alleles We know today that Mendel’s “factors” were genes, the part of the chromosome that governs the expression of a particular trait. A gene can occur in alternate forms called alleles.

Using letters to represent the different alleles, a purebred tall pea plant will have two uppercase letters, TT. Homozygous (the two alleles are the same). Heterozygous (the two alleles are different.) Each parent contributes 1 allele to the offspring.

Ratio of 3 dominant traits to 1 recessive trait. The dominant trait can be either homozygous or heterozygous. The recessive trait is always homozygous.

Punnett Squares A Punnett square is used to calculate the probability of inheriting a particular trait. The genotype is the genetic make-up of an organism. The phenotype is the appearance of the trait in an organism.

Review of Terms: Allele Gene Purebred Homozygous Heterozygous Punnett square Phenotype Genotype Dominant Recessive Hybrid Filial

Test Cross In order to determine the genotype of an individual, you must perform a test cross. A test cross involves crossing an individual of unknown genotype with a homozygous recessive individual. The offspring will exhibit certain phenotypes that will allow you to determine if your unknown is either homozygous dominant or heterozygous.

Example A woman has a father who died of Huntington’s disease. What is the probability that she will develop the symptoms of the disease? (Hint: Huntington’ disease is caused by a recessive allele.)

GRASP Given: Huntington’s is recessive Father died of Huntington’s therefore father must carry 2 recessive genes. Required: Probability daughter will develop Huntington’s disease.

Analysis: let “h” = Huntington’s allele let “H” = normal allele. Father’s genotype = “hh” father will give one allele to daughter. Substitute: If mother is “normal” then probability is: HH h h Hh mother father Solve: daughter will NOT develop Huntington’s but will carry the Huntington’s disease allele.

If mother is a carrier of the Huntington’s allele (Hh) then the probability is mother father hH h h Hh hh Hh hh 50:50 chance that the daughter will inherit a recessive gene from both parents and therefore she would develop Huntington’s disease.