Genetic Crosses Dominant and Recessive Genes Incomplete
Dominance
Pedigree Studies Sex Determination Gregor Mendel
Genetics
is the science of heredity and how new life changes and varies in
their characteristics. Sexual reproduction in humans involves 2 gametes; one male gamete, the sperm,
and one female gamete, the egg.
As was discussed in the cell
division webpage the gametes are haploid
(n). When fertilisation occurs the resulting fertilised egg is diploid (2n) Genetic variations occur
as a result of this union.
As was discussed in the heredity
webpage the physical characteristics of organisms are developed as the
protein builds up their bodies. These proteins are formed as a result of genes
carried on chromosomes.
In genetics genes are
represented by letters. There are usually 2 different types of the same
gene; one is dominant and one is recessive. An example of this
is: T is a gene for tall and t is a gene for short. The two versions
of the same gene are called alleles.
The two alleles are formed at the same position, or locus, on the chromosome.
REMEMBER: THE 2 ALLELES ARE GOTTEN FROM
THE 2 GAMETES OF SEXUAL REPRODUCTION. THAT IS WHY THEY CAN DIFFER.
IN ASSEXUAL REPRODUCTION THE NEW LIFE
IS ALWAYS IDENTICAL TO THE PARENT CELL BECAUSE THERE IS ONLY 1 PARENT.
Dominant
genes will always prevent the recessive gene from working. If a person
has 2 dominant alleles for tallness: T T
then he will be tall. If a person has 1 dominant and I recessive allele for
tallness: T t then he will be tall. The
only way the recessive gene will be expressed is if he has 2 recessive alleles
for short: t t then he will be short.
If the pair of genes controlling the characteristic has
identical alleles, TT or tt, we call it a homozygous
pair of genes. If the pair of genes controlling the characteristic are
different alleles, Tt or Cr, or Cw we call it a heterozygous pair of genes.
When we express the genotype
of a characteristic we state the pair of alleles. Genotype examples are TT, Tt, tt, BB, Bb, bb, etc.
When we state the physical characteristic expressed by the
genotype we are stating is phenotype.
The phenotype for TT is tall while
the phenotype for tt is short. These
phenotypes could vary because of environment
effects. This is especially true in terms of genotypes and phenotypes for
intelligence. The upbringing, environment, and education experiences greatly
affect the phenotype.
REMEMBER: THE 2 ALLELES ARE GOTTEN FROM
THE 2 GAMETES OF SEXUAL REPRODUCTION. THAT IS WHY THEY CAN DIFFER.
When working on genetic crosses you must state the capital letter which represents the
dominant allele and the small letter
that represents the recessive allele.
The following is an example of how to
work out genetic crosses:
|
A is a dominant characteristic. |
a is a
recessive characteristic. |
|
This bird has two genes for red feathers. Its genotype is AA. Its phenotype is RED |
This bird has two genes for blue feathers. Its genotype is aa. Its phenotype is BLUE |
This Punnett Square shows how we can diagram
the genes.
The orange bird has two
dominant A genes. We put two A s along the top of the square.
The blue bird has two
recessive a genes. We put two a
s down along the left side of the square.
All the offspring have
the genes Aa.
They will all have orange feathers (phenotype), but will carry a recessive gene for blue
feathers (genotype).The progeny are the offspring produced.
This is called the F1 generation progeny.
Now suppose that two individuals from the F1
generation become parents. Here they are!
The baby birds are called the F2 generation. You can see how their genes work
out. The offspring are coded in the squares. One bird will be orange with two AA genes.
Two birds will be orange with genes coded Aa.
One bird will be blue and will have two recessive aa
genes. Individual nests of birds may not turn out exactly like this, but if
there are many baby birds, they will work out genetically with the ratios 1:2:1.
Genotype: 1 AA, 2 Aa, 1 aa
Phenotype: 3 orange feathers, 1 blue feathers
Punnett
Square Animated Example
Incomplete
dominance is the situation where two different alleles are equally
dominant. When this occurs the heterozygous genotype that is produced is an
intermediate phenotype (blend)
between the two respective homozygous genotypes. This is also called codominance.
In this
example AA genotypes have red, Aa
genotypes pink and aa genotypes whitish flowers. Note that the heterozygous genotype Aa is a blend of red
and white.
Note: This
is the F1 generation.
In the F2 generation two pink flowers will
produce 2 pink phenotypes, 1 red phenotype, and 1 white phenotype.
A pedigree is a diagram showing the genetic history of a group of related organisms.
A pedigree showing the occurrence of a recessive
trait in three generations of a family
.
Circles in a pedigree represent females and squares
represent males. A horizontal line between a circle and a
square indicates a marriage or
partnership. Vertical lines indicate
the children from the marriage
or partnership. In this activity, a
filled-in circle or square shows that the individual has both alleles for the
trait. A half-filled-in circle or square indicates that the individual has one
recessive allele for the trait.
Our cells have 46 chromosomes. There
are 23 pairs. Remember, we get 23 from our mother and 23 from our father. We have
22 pairs of chromosomes called autosomes.
These are the chromosomes that control our body growth, enzymes, etc. We have 1 pair of sex chromosomes.
If the person is a male that pair is composed of an X and a Y chromosome. The male’s genotype is XY.
If the person is a female she will have a sex
chromosome composed of 2 Y chromosomes. The
female’s genotype is XX.
The
If
the child has a genotype of XX then it
becomes a girl. If the child has a genotype of XY then it becomes a boy.
As you can see the ratio of males to females is 1:1.
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Review Questions
1.
Which of the following is a possible abbreviation for a genotype?
A. BC
B. Pp
C. Ty
D. fg
2. What is the best way to determine the phenotype of the feathers on a
bird?
A. analyze the bird's DNA
(genes)
B. look at the bird's feathers
C. look at the bird's beak
d. examine the bird's droppings
3. Which of the following pairs is not correct?
A. kk = hybrid
B. hybrid = heterozygous
C. heterozygous = Hh
D. homozygous = RR
4. The genes present in an organism represent the organism's __________.
A. genotype
B. phenotype
C. physical traits
5. Which choice represents a possible pair of alleles?
A. k & t
B. K & T
C. K & k
D. K & t
6. How many alleles for one trait are normally found in the genotype of
an organism?
A. 1
B. 2
C. 3
D. 4
7. Which statement is not true?
A.
genotype determines phenotype
B. phenotype determines genotype
C. a phenotype is the physical appearance of a trait in an organism
D. alleles are different forms of the same gene
8. In the cross Yy x Yy, what percent of
offspring would have the same phenotype as the parents?
A. 25%
B. 50%
C. 75%
D. 100%
9. In a certain plant, purple flowers are dominant to
red flowers. If the cross of two purple-flowered plants produces some
purple-flowered and some red-flowered plants, what is the genotype of the
parent plants?
A. PP x Pp
B. Pp x Pp
C. pp x PP
D. pp x pp
Base questions
#10-13 on the following information:
A
white-flowered plant is crossed with a pink-flowered plant. All of the F1
offspring from the cross are white.
10. Which phenotype is dominant?
11. What are the genotypes of the original parent plants?
12. What is the genotype of all the F1 offspring?
13. What would be the percentages of genotypes &
phenotypes if one of the white F1 plants is crossed with a pink-flowered plant?
14. The outward appearance (gene expression) of a
trait in an organism is referred to as:
A. genotype
B. phenotype
C. an allele
D. independent assortment
15. In the homologous chromosomes shown in the
diagram, which is a possible allelic pair?
A. cD
B. Ee
C. AB
D. ee
16. The
phenotype of a pea plant can best be determined by:
A. analyzing its genes
B. looking at it
C. crossing it with a recessive plant
D. eating it
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Genetics Page
CORRECT ANSWERS FOR Q’S 1-9 ARE UNDERLINED
1. Which of the following is a possible abbreviation for a genotype?
A. BC
B. Pp
- genotypes are made up of 2 of the same letter (either 2 capital, 2
lowercase, or one of each)
C. Ty
D. fg
2. What is the best way to determine the
phenotype of the feathers on a bird?
A. analyze
the bird's DNA (genes)
B. look at the bird's feathers
- "phenotype of the feathers" means what the feathers look like, so
look at them
C. look at the bird's beak
d. examine the bird's droppings
3. Which of the following pairs is not
correct?
A. kk =
hybrid - Kk would be hybrid (one capital, one lowercase of the same letter)
B. hybrid = heterozygous
C. heterozygous = Hh
D. homozygous = RR
4. The genes present in an organism represent
the organism's __________.
A.
genotype
B. phenotype
C. physical traits
5. Which choice represents a possible pair of
alleles?
A. k &
t
B. K & T
C. K & k - allele means 2 forms of the same
gene. so this choice shows 2 forms of the same letter K or k
D. K & t
6. How many alleles for one trait are normally
found in the genotype of an organism?
A. 1
B. 2 - one
allele is inherited from each parent
C. 3
D. 4
7. Which statement is not true?
A.
genotype determines phenotype - (note that the environment does play a role in
influencing phenotype too)
B. phenotype determines genotype
C. a phenotype is the physical appearance of a trait
in an organism
D. alleles are different forms of the same gene - (see
question #5)
8. In the cross Yy x Yy, what
percent of offspring would have the same phenotype as the parents?
A.
25%
B. 50%
C. 75%
- in the completed p-square, 3 of 4 boxes will have at least 1 "Y",
producing the dominant phenotype (same as parents)
D. 100%
9. In a certain plant, purple
flowers are dominant to red flowers. If the cross of two purple-flowered
plants produces some purple-flowered and some red-flowered plants, what is the
genotype of the parent plants?
A.
PP x Pp
B. Pp x Pp - for any offspring to be recessive, each parent MUST have at least one
"p"
C. pp x PP - only one parent is purple, this CAN'T be an answer
D. pp x pp - neither parent is purple, this CAN'T be an answer
Base
questions # 10-13 on the following
information:
A
white-flowered plant is crossed with a pink-flowered plant. All of the F1
offspring from the cross are white.
10. Which phenotype is dominant? White.
11. What are the genotypes of the
original parent plants? WW (pure white) x ww (pink)
12. What is the genotype of all
the F1 offspring? Ww (white)
13. What would be the percentages
of genotypes & phenotypes if one of the white F1 plants is crossed with a
pink-flowered plant?
50% heterozygous white & 50%
homozygous recessive pink.
The cross for this question would be "Ww (white
F1) x ww (pink)".
The alleles of the white parent are above the
columns & those of the pink parent are in front of the rows. 2 of 4 boxes
(50%) are "Ww", which is heterozygous & would have the dominant
trait (white). The other 2 of 4 boxes (50%) are "ww", which is
homozygous recessive & would have the recessive trait (pink).
14. The outward appearance (gene
expression) of a trait in an organism is referred to as:
A.
genotype
B. phenotype
C. an allele
D. independent assortment
15. In the homologous chromosomes
shown in the diagram, which is a possible allelic pair?
A. cD
B. Ee- a possible allelic
pair but NOT SHOWN IN THE DIAGRAM, so this CAN'T be an answer
D. ee - an
"allelic pair" is always two forms of the same letter. In this
example they are two lowercase "e's".
16. The phenotype of a pea plant can best be
determined by:
A.
analyzing its genes
B. looking at it
C. crossing it with a recessive plant
D. eating it
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