GENETICS
Genetics is the study of inheritance (i.e. the study
of the passage of traits from one generation to another). Genetic
traits are carried on chromosomes, which are long molecules of a
nucleic acid called DNA. Each chromosome carries the genetic code
for many different traits. Each trait is at a different location on the
chromosome. We call these traits genes.
Human cells (except for
eggs and sperm) each have 46
chromosomes; this is called the diploid number. Gametes (sperm
and eggs) each have 23 chromosomes; this is called the haploid
number. When fertilization occurs (a sperm and an egg fuse
creating a fertilized egg called a zygote) the diploid number of 46 is
thereby restored. The abbreviation for diploid is 2N and for
haploid is N. The diploid chromosomes are found in pairs and are
actually two sets of genetic instructions, one from the mother (egg)
and the other from the father (sperm). We call these pairs of
chromosomes homologous chromosomes, because they carry genes
controlling for the same traits. If a chromosome contains a gene for
height, its homologue also contains a gene for height. Although
homologous chromosomes code for the same traits (e.g. - hair color)
they may not code for the same expression of a trait (e.g. - brown vs.
blond hair). The various forms of any one gene are called alleles
(using the previous example, blond and brown are both alleles of the
hair color trait). This leads to some terms:
Homozygous - refers to
homologous chromosome pairs
that include identical gene(s) or alleles (e.g. both for brown hair).
Heterozygous - refers to
pairs which are different
(e.g. 1 brown, & 1 blond)
Dominant - the prevailing
or most potent gene
Recessive - the
non-prevailing or less potent gene
Traits are abbreviated
with a single letter; a
capital letter indicates a dominant trait whereas a small letter
indicates a recessive trait. A recessive trait will not be
expressed if paired with a dominant trait; therefore recessive traits
are only expressed in the homozygous state.
An individual’s
actual genetic makeup, meaning
which genes are present is called genotype, whereas their outward
appearance due to how the genes are expressed is called their phenotype
(phenotype is what you see .. due to the genotype) . Using eye color as
an example: B=brown & b=blue. A person could have the following
genotypes:
BB = homozygous brown (or
you could say homozygous
dominant)
Bb = heterozygous brown
(or you could say
heterozygous for eye color)
bb = homozygous blue (or
homozygous recessive)
Note that a homozygous dominant person (BB)
has a different genotype
than a heterozygote (Bb), however they both have the same phenotype (=
brown eyes).
In humans one pair of the
23 homologous pairs are
sex chromosomes the other 22 pairs are called autosomes. Female
sex chromosomes are identical in appearance, code for the same traits,
and are designated X and X, while in the male the two sex chromosomes
are different and designated X and Y. Sex chromosomes like all
chromosomes code for many traits not just an individuals sex, however
the Y chromosome is very small and there are very few (~10) other
traits carried on the Y.
In the 1860’s an Austrian
monk named Gregor Mendel
worked out the basic principals of genetics in a series of experiments
crossing pea plants. Mendel developed three basic laws that
govern inheritance and hold true to this day.
1) Law of Segregation - In the cells of
individuals the genes (&
chromosomes) occur in homologous pairs. In the formation of
gametes (sex cells) the homologous chromosomes will separate, thus the
genetic traits become separated and the chromosome number is reduced
from diploid to haploid.
2) Law of Independent Assortment - When
chromosomes segregate, they do
so in a random fashion, meaning they separate independently of each
other.
3) Law of Recombination - Gametes (haploid)
unite at the time of
fertilization. This returns the chromosome number to the diploid
in the new generation, and often provides for different genetic
combinations than the previous generation.
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Usually, if a
person experiences
difficulty with genetic problems, it is because they fail to get the
gametes correctly.
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Genetics Problems:
1. Monohybrid cross - A
cross that deals with the inheritance of one single trait at a time.
a) Tall (T) is dominant over short
(t). If one crosses a homozygous tall plant
with a short plant, what will the F1 generation be like?
Provide the following genotypes:
Parents _______________ x _______________
Gametes _______ ______ x ______ ________
F1 Offspring _______________
Now, cross two of the F1
offspring.
Parents ____________ x _____________
Gametes _________ ________ _________ _________
F2 Offspring _________ _________ _______ ________
Give: Phenotypic Ratio ______________
Give: Genotypic Ratio _______________
In garden peas, some
peas have a smooth
(round) seed coat, others are wrinkled. If one crosses a
homozygous round (R) with a wrinkled (r), what will the F1 offspring be
like?
Provide the following genotypes:
Parents _______________ x _______________
Gametes ______________ x _______________
F1 Offspring ________________________
Now cross two of
the above F1 offspring.
Parents
__________________ x
__________________
Gametes _________________ x __________________
F1
Offspring _________________________
Give the phenotypic and
genotypic
ratios. ________________________
________________________
2. Incomplete Dominance - Sometimes one gene is not completely
dominant over its allele.
In certain flowering plants, red
(R) is
not completely dominant over white (r), and the heterozygote (Rr) is
pink.
Cross a red flower with a white
flower.
Parents __________________ x ___________________
Gametes _________ ________ x _________ ________
F1 _____________________ (Color: ______________)
Now, cross
two
F1 Offspring:
Parents __________________ x __________________
Gametes ________ _________ x ________ ________
F2 _________ _________ _________ ___________
Phenotypic Ratio: ___________________________
Genotypic Ratio: ____________________________
3. Test Cross
- If one needs to
determine the genotype of an individual, then cross the individual in
question to a known recessive. If the individual in question is
homozygous dominant, all the F1 offspring will be alike. If the
parent in question is heterozygous, then the F1 offspring will be in a
ratio of 1:1 between the test parent and the known recessive.
A black cat
can be B/B or B/b, while a
white cat (b/b) is the known recessive. Do the following
testcrosses:
Parents (Homozygous Black) __________ x _________(white)
Gametes
__________ x _________
F1
________________________
Ratio
______________________
Parent (Heterozygous
Black) ________ x _________(white)
Gametes ___________ ___________ x __________
F1 ______________ _____________
Ratio _____________________
4. Dihybrid Cross -
Two different types of
traits, located on different pairs of homologous chromosomes are
inherited independently, but at the same time.
Round (R) is
dominant over wrinkled (r)
coat, while yellow (Y) is dominant over green (y). Cross a
homozygous round, yellow with a wrinkled green.
Parents _______________ x ______________
Gametes ______________ x ______________
F1 ________________
Phenotype _____________________________
Genotype _____________________________
Cross
two of the F1 offspring.
Parents ________________ x
________________
Gametes _______ _______ x ________ _______
________ _______ ________ _______
Give the Phenotypic Ratio: ________________
5. Sex Ratio - Sex
of an individual is
determined by the sex chromosomes, and there should be a ratio of one
female for each male. Prove if this is correct for people.
Male XY, Female XX.
Parent (Father) ___________ x ___________(Mother)
Gametes _____________
_____________
_____________ _____________
F1
_____________ _____________
Now, cross two
F1 individuals.
Parent (Father) ___________ x ____________(Mother)
Gametes ____________ x
____________
F2
____________ ____________
6. Sex-Linked
Characteristics - There are many traits
carried on the X sex chromosome, while for all practical purposes,
there are no traits carried on the Y sex chromosome.
Colorblindness in people is a
sex-linked characteristic, and it is a recessive trait. If a
heterozygous, normal vision, female (carrier) marries a normal vision
male, what type of offspring would be expected?
Parents (Father) _______________ x
_____________(Mother)
Gametes _________ _________ x _________ _________
F1 __________ __________ __________ _________
Phenotypic Ratio (All children) ____________________
Genotypic Ratio (Males) _________________________
Genotypic Ratio (Females) _______________________
If a "carrier" normal
vision female marries a colorblind male, what type of offspring would
be expected?
Parents (Father) ____________ x ____________(Mother)
Gametes ________ ________ x __________ _________
F1 _____________ _________ ___________ _________
Phenotypic Ratio (All children) ___________________
Genotypic Ratio (Males) ____________________
Genotypic Ratio (Females) __________________
7. Multiple
Alleles - when a single
trait such as blood type has more than two factors, but only two
factors may function at a given time.
People have four
phenotypes of blood-- A, B, AB, and O. However there are 3 alleles
involved (A, B, & O), A & B are not dominant over each other
(codominant), but they are each dominant over the “O” allele.
A may be A/A, A/O
B may be B/B, B/O
AB is A/B
O is O/O
If a
heterozygous A female marries a heterozygous B male, what type of
offspring would be expected?
Parents (Father) __________ x __________(Mother)
Gametes _________ _________ ________ _________
F1 _________ _________ _________ __________
Give Genotype ________ ________ ________ ________
8. Sex-Influenced
Characteristics - Genes in
which dominance varies depending on the sex of the individual. A
classical example in people is baldness.
Genotype
Men Women
B/B
Bald
Bald
B/b
Bald
Non-bald
b/b
Non-bald Non-bald
NOTE: In working these problems, one must
include the sex chromosomes
in order to keep the genotypes separate.
A non-bald, normal vision
man marries a non-bald,
normal vision woman whose father was color-blind and whose mother was
bald. What kinds of offspring may they have, and in what
proportions?
Parents (Father) ______________ x _____________(Mother)
Gametes _______ _______ x _______ _______ ______ _______
F1 Use Punnett Square
Give genotypes for each offspring.
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Genetics review
*Sex cells = gametes ( sperm & ovum), they have one-half the
normal chromosome number, = 23 chromosomes, this is called
haploid
(symbol = n) (so gametes have 22 autosomes & 1 sex chromosome)