GENETICS
Genes are lengths of DNA, found at specific site along chromosomes
Genes come in pairs, residing in pairs of chromosomes
During Meiosis, homologous chromosomes line up next to each other then separate, with each member of a pair
ending up in a different egg or sperm cell
Genes are passed on intact when the chromosomes in sperm join with the chromosomes in egg.
Alleles are alternative forms of the same gene
Traits are character variations.
Phenotype is the physical characteristic or “trait.
Genotype is the genetic basis for the trait it represents the combination of alleles you inherit
 Homozygous Dominant
 Homozygous Recessive
 Heterozygous.
Gregor Mendel is the father of genetics. His discoveries in genetics were based on work with Pisum sativa (garden
pea).
Characteristics of Pisum sativa
 They can self pollinate
 They can be cross pollinated
 Many of the measurable traits were dominant or recessive
Mendels First Experiment was a Monohybrid Cross.
1. Parental (P) generation. He began by crossing plants that “bred true” for one trait. Individuals
true-breeding for a trait inherited the same two allele for a trait. Female Plants true breeding for yellow seed
are homozygous dominant for the dominant allele Y (YY). Male Plants true breeding for green seed are
homozygous recessive for the recessive allele y (yy). During meiosis the two alleles for color, which lie on
separate chromosomes, separate. Each female gamete (egg) bears a Y and each male gamete (sperm) bears y.
2. Filial (F)1 generation represents the offspring of the P generation. When the egg bearing the Y
joins with the sperm bearing the y, the single possible outcome for their offspring is Yy (heterozygous).
Because Y (yellow) is dominant over y (green), the result is that all the offspring in the F1 generation are
yellow, because they all contain a Y allele.
3. Filial (F)2 generation represents the off spring of the F1 generation. In this cross, a male
heterozygous for yellow (Yy) is crossed with a female, also heterozygous for yellow (Yy). There are three
possible genotypes (YY, Yy, and yy). And two possible phenotype yellow or green seed. Mendel found that
75% of the offspring had yellow seeds and 25% had green seeds. The genotypes that matched this phenotype is
25% were YY, 50% were Yy and 25% were yy .
THE EXPECTED PHENOTYPIC RATIO, IN THE F2-GENERATION, IN A MONOHYBRID CROSS IS 3:1
DOMINANT TO RECESSIVE. The monohybrid cross demonstrated Mendel's principle of segregation, that during
gamete formation the alleles in each gene segregate and pass randomly into gametes.
Mendel’s Second Experiment was a Dihybrid (2 traits) Cross.
1. Pgeneration was a cross with a plant homozygous for round-yellow seeds (genotype RRYY) and
another plant homozygous for wrinkled-green seed (genotype rryy)
2. F1 generation all offspring had round-yellow phenotype and genotypes were all heterozygous for two
traits (RrYy)
3. Crosses between offspring from the F1 generation each heterozygous for each trait. The genotypes in the
cross are (RrYy and RrYy). The phenotypic ratios of the offspring in the F2 generation are 9:3:3:1. Nine offspring
have round and yellow seeds, 3 have round and green seeds, 3 have wrinkled and yellow seeds and 1 has wrinkled
and green seeds.
Mendel deduced that an organism has two genes (we now call alleles) for each inherited characteristic
Mendel proposed two principles
1. The Principle of Segregation: states that pairs of genes segregate (separate) during gamete formation and fusion
of gametes at fertilization pairs genes once again. Demonstrated in a monohybrid cross.
2. The Principle of Independent Assortment: Each pair of alleles segregates independently during gamete formation.
Demonstrated during the dihybrid cross. Independent (Random) Assortment of Alleles can be demonstrated in
Meiosis.
The testcross is used to determine unknown genotypes. It involves a cross between one individual having recessive
phenotype (known genotype) and another have a dominant phenotype (with unknown genotype). The phenotypic
ratio reveals the genotype of the parent with the dominant phenotype. Give it a try with the following two crosses.
You can determine to genotype of the person with the dominant trait using the test cross.
Inherited disorders in humans controlled by a single gene can be recessive or dominant
Recessive Disorders:Albinism, Deafness, Cystic fibrosis, Sicke-cell disease and Tay-Sachs
Dominant Disorders: Achondroplasia, certain types of Alzheimer’s disease, Huntington’s Disease and
Hypercholesterolemia
VARIATIONS ON MENDEL’S PRINCIPLE
I. Incomplete Dominance  produces intermediate phenotypes. These traits can not completely mask out the
effects of the recessive trait. Examples are Sickle-cell, hypercholesterolemia, and In plants it’s the pink flower color
in snapdragon.
II. Multi-allelic: In typical Mendelian Inheritance 2 alleles determine phenotype. In this case there are > 2 alleles for
a trait in a population. Multiallelic inheritance produces a range in phenotypes. In Plants, the varigation patterns on
clover leaves “Chevron” is determined by a gene with multiple alleles. In humans, there are four blood groups A, B,
AB and O. These groups are derived from 3 alleles: the A, B and O allele.
III. Pleiotropy: One gene influences several characteristics. Study the pleitrophic effects of the S-gene in tobacco.
The S gene affects the corolla, anther, calyx, leaf, and capsule. In humans sickle cell trait produces pleiotrophic
effects.
IV. Multi-genic: In this case > 2 alleles determines phenotype. Multiallelic inheritance produces a range in
phenotypes. Multialleles determines Skin Color and height.
V. SEX-linked genes are more common in boys. The pair of sex chromosomes determines sex in many species. In
humans XX female and XY  male. Y chromosomes has genes for testes development, without Y, ovaries
develop. All genes on sex chromosomes are considered sex-linked. In human and fruit fly, traits on the X
chromosome determine sex-linked inheritance. Three examples of sex-linked disorders : Duchene muscular
dystrophy, color-blindness and hemophilia.
For a female to have either disorder she must inherit a defective allele on both X chromosomes (one from each
parent); however, a boy will inherit the disorder on the X derived from mom.
GENETIC SCREENING: testing people for alleles associated with a particular disorder.
GENES ON THE SAME CHROMOSOME TEND TO BE INHERITED TOGETHER
CROSSING OVER PRODUCES NEW COMBINATIONS OF ALLELES AND IS USED TO MAP GENES
It is usually revealed when the phenotypic ratio in the dihybrid cross is something other than 9:3:3:1.
CHROMOSOMES DETERMINE SEX IN MANY SPECIES. Plants that are dioecious separate sexes on two
different plants would have sex chromosomes. The variation in gender in plants is diverse. Perfect flowers have are
on the same plant. These flowers have both male and female parts.
A Pedigree is a chart that can be used to track genetic traits in humans
This pedigree is sex linked.
A few practice problems:
1. In humans, a normal skin pigmentation (N) is dominant to the albino condition (n) [all skin coloration is gone].
A normally pigmented man, whose father was an albino, marries an albino woman. What are the chances of this
couple having an albino child?
2.
In a child custody case, Sally Jones is absolutely certain that the man she was living with, John Smith, is not the
father of the child they are fighting over. Sally Jones has type A blood. Her mother is type O and her father is
type
AB. John Smith has type A blood. Both of his parents have type AB blood. If the son who is in dispute has type
O blood, explain why John Smith could not be his father. (Hint: determine the genotypes of all of the members of
the family.)
3. In humans, brown eyes (B) are dominant over blue (b)*. A brown-eyed man marries a blue-eyed woman and
they have three children, two of whom are brown-eyed and one of whom is blue-eyed. Draw the Punnett square that
illustrates this marriage. What is the man's genotype? What are the genotypes of the children?
4. If the man has brown eyes, but has a blue-eyed child what must his genotype be?
(if you don�t understand why, review the testcross problem)
5. Testcross:
In dogs, there is an hereditary deafness caused by a recessive gene, �d.� A kennel
owner has a male dog that she wants to use for breeding purposes if possible. The dog
can hear, so the owner knows his genotype is either DD or Dd. If the dog�s genotype
is Dd, the owner does not wish to use him for breeding so that the deafness gene will
not be passed on. This can be tested by breeding the dog to a deaf female (dd). Draw
the Punnett squares to illustrate these two possible crosses. In each case, what
percentage/how many of the offspring would be expected to be hearing? deaf? How
could you tell the genotype of this male dog? Also, using Punnett square(s), show how
two hearing dogs could produce deaf offspring.
6. A deaf female is genotype dd. What kind(s) of gametes (eggs) can she produce?
7. Sex-Linked Genes:
In humans, the genes for colorblindness and hemophilia are both located on the X chromosome with no
corresponding gene on the Y. These are both recessive alleles. If a man and a woman, both with normal vision,
marry and have a colorblind son, draw the Punnett square that illustrates this. If the man dies and the woman
remarries to a colorblind man, draw a Punnett square showing the type(s) of children could be expected from her
second marriage. How many/what percentage of each could be expected?
8. In Mendel's experiments, the spherical seed character (SS) is completely dominant over the dented seed character
(ss). If the characters for height were incompletely dominant, such that TT are tall, Tt are intermediate and tt are
short, what would be the phenotypes resulting from crossing a spherical-seeded, short (SStt) plant to a dentedseeded, tall (ssTT) plant?
9. If Mendel's crosses between tall, spherical-seeded plants and short, dented-seeded plants had produced many
more than 1/16 short, dented-seeded plants in the F2 generation, he might have concluded that: