Module 03: Cell Reproduction
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Genetics studies genes and inheritance. Mendel studied pea plants because they were easy to grow, had contrasting traits, produced many offspring, and had short life cycles. Controlling which plants reproduced let him connect parental traits with offspring traits.
Peas can self-pollinate: sperm from pollen fertilizes an egg from the same plant. Mendel prevented this by removing the pollen-producing parts, then supplied pollen from a different plant. This cross-pollination let him compare parents with contrasting traits.
Self-pollination still combines sperm and egg and is sexual reproduction. It uses gametes from the same plant. Self-pollination can produce offspring with different allele combinations, so it does not by itself guarantee a clone.
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P labels the original parents. F1 labels their offspring. F2 labels offspring of the F1 plants. In the purple-and-white flower experiment, the F1 flowers were purple; white reappeared in about one quarter of F2. That reappearance matters: the white-flower allele was still present.
Alleles are different versions of a gene. In Mendel’s simple dominant/recessive pattern, one dominant allele is enough for its trait to appear. The recessive trait appears when both alleles are recessive. Dominant does not mean most common in a population.
Genotype names the allele combination. Phenotype is the observable trait. Homozygous means two matching alleles; heterozygous means two different alleles. Two plants can look purple while carrying different allele combinations, so appearance alone does not always reveal genotype.
For a simple dominant/recessive gene, two dominant alleles and one dominant plus one recessive allele can produce the same dominant phenotype. Two recessive alleles produce the recessive phenotype. Genotype is the combination; phenotype is what is expressed.
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The law of segregation says the two alleles separate during gamete production. A heterozygous parent can make gametes carrying either allele. Fertilization brings one allele from each parent together again, restoring the pair in the offspring.
The F1 hybrids can carry the recessive allele while showing the dominant trait. Their gametes separate the alleles. When two recessive alleles come together in an F2 plant, the recessive trait appears. The roughly 3:1 phenotype pattern describes many offspring, rather than an exact guarantee for every group of four.
Mendel’s four conclusions connect: genes have alternative versions called alleles; offspring inherit one allele from each parent; one allele can dominate expression in a heterozygote; the pair separates when gametes form. These conclusions explain the observed generations without blending away the recessive allele.
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Choose an answer, then use the explanation to check your reasoning.
