Objectives
🧬 Explain the significance of Mendel’s experiments to the study of genetics.
🧬 Summarize the law of segregation and law of independent assortment.
🧬 Predict the possible offspring from a cross using a Punnett square.
Vocabulary
genetics: science of heredity
allele: alternative form that a single gene may have for a particular trait.
dominant: Mendel’s name for a specific trait that appeared in the F1 generation.
recessive: Mendel’s name for a specific trait hidden or masked in the F1 generation.
homozygous: organism with two of the same alleles for a specific trait
heterozygous: organism with two different alleles for a specific trait.
genotype: an organism’s allele pairs.
phenotype: observable characteristic that is expressed as a result of an allele pair.
law of segregation: Mendelian law stating that two alleles for each trait separate during meiosis
hybrid: organism heterozygous for a specific trait.
law of independent assortment: Mendelian law stating that a random distribution of alleles occurs during the formation of gametes.
How Genetics Began
Gregor Mendel, an Austrian Monk, lived in the 1800s. He experimented with pea plants in the monastery gardens.
Pea plants usually reproduce by self-fertilization. This means that the female gamete is fertilized by a male gamete in the same flower. Mendel discovered a way to cross-pollinate peas by hand. He removed the male gametes from a flower. He then fertilized the flower with the male gamete from a different flower.
Through these experiments, Mendel made several hypotheses about how traits are inherited. In 1866, he published his findings. That year marks the beginning of the science of genetics, the science of heredity. Mendel is called the father of genetics.
The Inheritance of Traits
Mendel used true-breeding pea plants (plants whose traits stayed the same from generation to generation). Mendel studied seven traits—flower color, seed color, seed pod color, seed shape, seed pod shape, stem length, and flower position.
What did Mendel find when he crossed pea plants with different traits?
Mendel called the original plants the parent, or P, generation. The offspring were called the F1 generation. The offspring of the F1 plants were called the F2 generation.
In one experiment, Mendel crossed yellow-seeded and green-seeded plants. All the F1 offspring had yellow seeds. The green-seed trait seemed to disappear.
Mendel allowed the F1 plants to self-fertilize. He planted thousands of seeds from these plants. He saw that in these offspring, the F2 generation, three-fourths of the plants had yellow seeds and one-fourth had green seeds, a 3:1 ratio. Mendel performed similar experiments for other traits. Each time, he observed the same 3:1 ratio.
The results of Mendel’s cross involving true-breeding pea plants with yellow seeds and green seeds are shown here.
How did Mendel explain his result?
Mendel proposed that there were two forms of each trait, and each form was controlled by a factor, which is now called an allele. An allele is a different form of a gene passed from generation to generation. Yellow-seed plants have a different allele than green-seed plants.
Mendel proposed that each trait was controlled by two alleles. The dominant form is the version of the trait that appears in the F1 generation. The recessive form is the version that is hidden in the F1 generation.
How does dominance work?
When written, the dominant allele is represented by a capital letter (X). The recessive allele is represented by a lowercase letter (x).
An organism is homozygous if both alleles for a trait are the same. The organism is heterozygous if the alleles for a trait are different. In heterozygous organisms, only the dominant trait can be seen. Dominant alleles mask recessive alleles.
How do genotype and phenotype differ?
It is not always possible to know what alleles are present just by looking at an organism. A yellow-seed plant could be homozygous (YY) or heterozygous (Yy). An organism’s allele pairs are called its genotype. The expression of an allele pair, or the way an organism looks or behaves, is called its phenotype.
What is the law of segregation?
Recall that the chromosome number is divided in half during meiosis. The gametes contain only one of the alleles. Mendel’s law of segregation states that the two alleles for each trait separate from each other during meiosis and then unite during fertilization. When parents with different forms of a trait are crossed, the offspring are heterozygous organisms known as hybrids.
A cross which involves hybrids for a single trait is called a monohybrid cross. Mono means one. The offspring of the cross have a phenotypic ratio of 3:1.
How are two or more traits inherited?
Mendel also performed dihybrid crosses, crossing plants that expressed two different traits. Mendel crossed yellow, round-seed plants with green, wrinkle-seed plants. Round seeds are dominant to wrinkled, just as yellow color is dominant to green. He wondered whether the two traits would be inherited together or separately. Members of the F1 generation are dihybrids because they are heterozygous for both traits.
Mendel found that the traits were inherited independently. Members of the F2 generation had the phenotypic ratio of 9:3:3:1 (9 yellow round seeds, 3 green round, 3 yellow wrinkled, and 1 green wrinkled). From experiments with dihybrid crosses, Mendel developed the law of independent assortment, which states that alleles distribute randomly when gametes are made.
Punnett Square
In the early 1900s, Dr. Reginald Punnett developed a square to predict possible offspring of a cross between two known genotypes. Punnett squares are useful for keeping track of genotypes in a cross.
What information does a Punnett square contain?
A Punnett square can help you predict the genotype and phenotype of the offspring. The genotype of one parent is written vertically, on the left side of the Punnett square. The genotype of the other parent is written horizontally, across the top. A Punnett square for a monohybrid cross contains four small squares. Each small square represents a possible combination of alleles in the children.
The Punnett square below shows the results of Mendel’s experiment with seed color. The Punnett square shows that four different genotypes are possible—one YY, two Yy, and one yy. The genotypic ratio is 1:2:1.
Summary
🧬 The study of genetics began with Gregor Mendel, whose experiments with garden pea plants gave insight into the inheritance of traits.
🧬 Mendel developed the law of segregation and the law of independent assortment.
🧬 Punnett squares help predict the offspring of a cross.
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