Download Heredity Notes/Punnett squares

A. History of Heredity Studies
1) Gregor Mendel is considered
the father of genetics.
2) He performed experiments
on pea plants to find out why
living things possessed different
traits.
3) Some traits of pea plants
that he observed are as follows:
a) plant height (tall & short)
b) seed shape (round &
wrinkled)
c) seed color (yellow & green).
B. Key Words:
1) Heredity: the passing on of
traits from parents to offspring.
2) Genes: segments of DNA
located on the chromosomes that
code for a particular trait.
3) Traits: physical or behavioral
characteristics which are
inherited from parents.
4) Alleles: alternative forms of
a gene that govern the same
trait.
5) Recessive trait: a trait that
is visible only when two recessive
alleles for a trait are inherited.
6) Dominant trait: the trait
observed when at least one
dominant allele for a trait is
inherited.
7) purebred: an organism that
inherits the same alleles for a
particular trait.
8) hybrid: an organism that
inherits two different alleles for
a trait.
9) Punnett square: a tool used to
visualize all of the possible
combinations of alleles from
parents.
10) genotype: the inherited
combination of alleles.
11) phenotype: an organism’s
inherited appearance.
12) probability: the chance of an
event or combination of alleles
occurring and hence result in the
organism’s inherited traits.
13) P Generation: parents of an
organism.
14) F1 Generation: the offspring
from the P Generation.
15) F2 Generation: the offspring
from the F1 Generation or
grandchildren of the
P Generation.
16) homozygous: same as
purebred genotype (e.g. TT, tt, WW).
17) heterozygous: same as
hybrid genotype (e.g. RW, Tt).
C. Probability:
1. Determining the chance of a
trait appearing in an individual.
2. Use the Punnett Square to
determine the chance of a
particular trait appearing.
Some practice problems follow.
A plant that produces only round
seeds, RR and a plant that produces
only wrinkled seeds, rr were crossed.
Find the following:
The percent hybrid?
The percent purebred?
The percent having round seeds?
The percent having wrinkled
seeds?
R
R
r
Rr
Rr
r
Rr
Rr
A plant that produces only round
seeds, RR and a plant that produces
only wrinkled seeds, rr were crossed.
Find the following:
The percent hybrid?
100%
The percent purebred? 0%
The percent having round seeds?
100%
The percent having wrinkled
seeds?
0%
R
r
Rr
r
Rr
R
Rr
Rr
Two hybrid plants with round
seeds, Rr are crossed. The dominant
allele R, produces round seeds. The
recessive allele r, produces wrinkled
seeds. Find the following:
The percent hybrid?
The percent purebred?
The percent having round seeds?
The percent having wrinkled
seeds?
R
r
R
RR
Rr
r
Rr
rr
Two hybrid plants with round
seeds, Rr are crossed. The dominant
allele R, produces round seeds. The
recessive allele r, produces wrinkled
seeds. Find the following:
The percent hybrid? 50%
The percent purebred? 50%
R
r
R
RR
Rr
r
Rr
rr
The percent having round seeds?
75%
The percent having wrinkled
seeds?
25%
General Rules for filling in Punnett
Squares:
1) Each letter fills in the two
boxes immediately below or to the
right of them.
2) lower case letters are
written after UPPER CASE letters
no matter which was on top or the
side.
(e.g. Rr )
Four steps to solving probability
problems:
STEP 1: Choose letters to represent
the alleles of the parents being
crossed.
STEP 2: Write the genotypes of the
parents being crossed.
STEP 3: Construct a Punnett Square.
STEP 4: Answer the questions.
R
R
r
Rr
Rr
r
Rr
Rr
D. Complete Dominance vs.
Incomplete Dominance genes:
Complete Dominance:
a) condition in which dominant
genes completely hide or mask
recessive genes.
b) the same letter is used to
represent both the dominant and
recessive forms of a gene.
c) but, capital letters are used to
represent dominant genes and lower
case letters represent recessive
genes.
Incomplete Dominance:
a) condition in which dominant
genes incompletely hide other genes
and results in a “blending” of traits.
b) different capital letters are
used to represent different forms of
a gene (e.g. R = red, W = white).
c) a hybrid is represented by a
mixture of capital letters (e.g. RW).
References:
Holt, Rinehart & Wilson Science and Technology textbook, 2001, California Life
Science, pp. 130-137.
Prentice Hall textbook, 1993, Heredity.