Name: ____________________________________ Period: ________ Date: _______________
Gregor (Johann) Mendel’s Peas & the Law of Segregation
Mendel’s Law of Segregation explains how the sorting of “factors” during meiosis, and their
subsequent recombination during fertilization, can account for the sudden reappearance of
recessive traits in the second generation of hybrid offspring
The discrete inheritable units Mendel called “factors” are now called genes. We now know
that genes are lengths of the DNA molecules that carry the manufacturing instructions for one
protein and are arranged on a chromosome like beads on a string. The varieties of each plant
character Mendel observed are due to the presence of “alleles”, or alternative forms of the same
gene. Alleles differ from one another in just a few nucleotide bases; so they correspondingly code
for slightly different sequences of amino acids. They are found on the exact same position along
homologous chromosomes: they are said to occupy the same gene locus. Humans and sweet peas,
like all diploid organisms, possess two alleles for every trait. One was contributed by the female
parent, the other, by the male parent. Mendel invented the system of referring the alleles with
letters: capital letters stand for the dominant allele and small letters stand for the recessive
alleles. On this worksheet, the example of seed shape is used: “R” stands for the allele for smooth
seeds, and “r” represents the alleles for wrinkled seeds.
It is helpful at this point to master a few more new terms. “Genotype refers to the genetic
makeup of an individual for any given trait. If you have inherited the same alleles for a trait from
both parents, you genotype is homozygous for that trait. If you have inherited a different allele
from each parent, your genotype for that trait is heterozygous. “Phenotype” refers to the outward
appearance of the individual or the observable expression of the genotype. For example, pea plants
with smooth seed phenotype may have either of two genotypes “RR” or “Rr”. The genotype “Rr” is
heterozygous: one parent contributed the allele “R” and the other, the allele “r”. Since the “R’ allele
for smooth is dominant to the “r” allele for wrinkled seeds, the phenotype of both these genotypes
is smooth seeds. Below the parent plants, the pairs of homologous chromosomes that carry alleles
for seed shape in each individual are shown. One chromosome in each pair is dotted and one is left
plain to remind you that one is contributed by each parent. The alleles for the seed shape gene are
located in the same position on four chromosomes. Each of the parents has a homozygous genotype.
The female plant is homozygous for the smooth trait, and the male plant has a homozygous
recessive genotype possessing two alleles for wrinkled seeds. Remember: as homologous
chromosomes segregate, they carry with them all the genes along their length. Now we can see why
Mendel observed that 100 percent of the first generation of hybrid plants has smooth seeds. Each
pea plant contains a recessive allele for wrinkled seeds, but the allele for smooth seeds is
completely dominant and prevents the wrinkled allele from being expressed in the phenotype.
Observe what happens if this first generation of heterozygous genotypes is allowed to
interbreed. Gametes are produced and recombined, as the arrows indicate. Random mating produces
plants with three different genotypes, one of which is a recessive homozygote, “rr”. This one
genotype causes the wrinkled phenotype to reappear in this generation of offspring. Mendel
harvested over seven thousand peas and observed the ratio of three smooth peas to every one
wrinkled pea. A phenotypic ratio of 3:1. Through this observation deduced that maternally and
paternally derived “factors” segregate into separate gametes. The Augustinian monk knew nothing
of centrioles or meiotic spindles, but with our more sophisticated understanding of cellular division,
we can trace the mechanisms for his observed patterns of inheritance to the precise manner by
which chromosomes are sorted into daughter cells during meiosis. He is, indeed, THE FATHER OF
MODERN-DAY GENETICS!
DISCUSSION QUESTIONS:
1. The discrete inheritable units (that Mendel called “factors”) are today known as: ___________
2. What is an “ALLELE”? (Briefly explain) ___________________________________________
A. How do “alleles” differ, one from the other? _________________________________
____________________________________________________________________
B. “Alleles” are found on the exact same position along homologous (like-pairs)
chromosomes……True…..or……False? ________________
3. Humans, Edible pea plants…….in fact ALL Diploid Organisms posses ______(#) alleles for every
hereditary trait they have. One gene is contributed by the ___________ parent, while the other
gene is contributed by the _____________ parent.
4. Mendel invented a system of symbols that refer to alleles by using letters from the alphabet.
A. CAPITAL LETTERS…(ie: “A”, “T”, “R” etc) represent _______________alleles
B lower case letters…….(ie: “a”, “t”, “r” etc) represent _______________alleles
5. What term best fits the simple definition:
A. the “genetic make-up” or gene picture for a specific trait?_________________
B. the physical, outward appearance (observable) for a specific trait:___________
6. “Circle” the genotype from the list:
Brown…Curly…Wrinkled…#W23…Freckles...BB…Taster
7. Below, use a line to match the following GENOTYPES (symbols) with the appropriate word
description:
RR
Rr
Rr
Homozygous recessive
Homozygous dominant
Heterozygous
8. ON THE DIAGRAM… MENDEL’S PEAS: LAW OF SEGREGATION, COLOR THE FOLLOWING
A. The TITLE plus ALL parts that have a “STAR” ………………………….. DARK GREEN
B. ALL LABELS that are identified by a BLACK DOT ………………………BROWN
C. The PARTS, SYMBOLS, and the LABELS that are identified by:
…the letter “A” …………….. PURPLE
…the letter “B” ……………...DARK ORANGE
…the letter “C” ……………...DARK YELL0W
…the letter “D” ……………...DARK BLUE
…the letter “E” ………………LIGHT GREEN
D. Refer to the diagram’s “Homologous Chromosomes”. NEATLY,
“circle” the…two dominant alleles…(“R” & “R”) on the chromosomes; then
“circle” the…two recessive alleles…(“r” & “r”) on the chromosomes