Title: GENETICS AND HEREDITY
1GENETICS AND HEREDITY
2Heredity and Genetics
- Heredity is the passing of physical
characteristics from parents to offspring. - Genetics is the scientific study of heredity.
3Mendel
- Gregor Mendel, an Austrian monk of the nineteenth
century, made the discoveries that is the
foundation of our knowledge of genetics.
4Mendels Experiments
- He did his experiments because he wondered why
pea plants had different characteristics. - Tall and short plants
- Green and yellow seeds
- Round (smooth) and wrinkled seeds
- Each different form of a characteristic is called
a trait.
5Mendels Experiments
- Fertilization is the process where an egg cell
and a sperm cell join together. - Pollination is the process of the pollen reaching
the pistil of a flower. - Pea plants are usually self-pollinating, meaning
the pollen of a flower lands on the pistil of the
same flower. - Mendel developed a method of cross-pollination.
- He removed pollen from the flower of one plant
and then brushed the pollen onto a flower on a
second plant.
6Crossing Pea Plants
- Mendel decided to cross plants with opposite
traits, for example tall and short plants. - He began his experiments with purebred plants.
- Purebred organisms are the offspring of many
generations that have the same trait. - Example Purebred short plants always come from
short parent plants. - Purebred individuals are also called true
breeding individuals.
7The F1 Offspring
- In Mendels experiments, the purebred parent
plants are called the parental generation or P
generation. - Example Mendel crossed a purebred tall plant
with a purebred short plant. - The offspring of the P generation are called the
first filial (Latin for daughter or son), or F1
generation. - Example In Mendels F1 generation, all of the
plants were tall. - Even though one of the parents was short, that
trait seemed to disappear in the F1 generation.
8The F2 Generation
- Mendel let the fully grown F1 plants to
self-pollinate. - The second filial, or F2 generation were a mix of
tall and short plants. - The short trait reappeared even though none of
the parents were short. - After counting the F2 plants, Mendel noted that ¾
of the plants were tall and ¼ of the plants were
short.
9Experiments with Other Traits
- Mendel did hundreds of crosses looking at other
traits. - In all of his crosses, only one form of the trait
appeared in the F1 generation, but that trait
reappeared in the F2 generation in about ¼ of the
plants.
10Dominant and Recessive Alleles
- Because of his experiments, Mendel concluded that
individual factors must control the inheritance
of traits. - He also reasoned that the factors that control
each trait exists in pairs, one factor from each
parent. - Based on the results of his experiments, Mendel
concluded that one factor in each pair can mask,
or hide, the other factor. - Example The tallness factor masked the shortness
factor.
11Genes and Alleles
- Today, scientists call the factors that control a
trait a gene. - The two different forms of a gene are called
alleles. - Each pea plant inherits one allele from each
parent. - A pea plant could inherit 2 tall alleles, 2 short
alleles, or 1 of each.
12Genes and Alleles
- An organisms traits are controlled by the
alleles it inherits from its parents. - Some alleles are dominant.
- Dominant alleles are those whose trait always
shows up in the organism when that allele is
present. - Other alleles are recessive.
- Recessive alleles are those whose traits are
hidden whenever the dominant allele is present. - Recessive traits only show up if the organism
does not have the dominant allele. In other
words, the organism has two recessive alleles.
13Genes and Alleles
- In Mendels crosses, the allele for tall plants
is dominant over the allele for short plants. - Only plants that inherit two short alleles will
be short. Plants that receive one or two dominant
alleles will be tall.
14Alleles in Mendels Crosses
- In Mendels experiments, the purebred tall plants
had 2 alleles for being tall, while the purebred
short plants had 2 alleles for being short. - All of the plants from the F1 generation received
one tall allele and one short allele. - Organisms that has two different alleles for a
trait is called hybrid. - All of the hybrid plants were tall because they
received 1 tall and 1 short allele, but the tall
is dominant over the short.
15Alleles in Mendels Crosses
- When the F1 plants self-pollinated, some of the
F2 plants received two dominant alleles for
tallness. - These plants were tall.
- Other F2 plants received one dominant and one
recessive allele. - These plants were tall.
- The rest of the F2 plants received two alleles
for shortness. - These plants were short.
16Symbols for Alleles
- Letters are used to represent alleles.
- Dominant alleles are represented by capital
letters. - The tall allele would be T.
- Recessive alleles are represented by lowercase
letters. - The short allele would be t.
- The alleles an organism receives for a trait are
represented by a combination of letters. - The combination of alleles possible for pea
plants are TT, Tt, and tt.
17Homozygous and Heterozygous
- An organism is said to be homozygous for a trait
if both alleles are identical. - Example TT and tt are homozygous allele
combinations. - TT is homozygous dominant.
- tt is homozygous recessive.
- An organisms is said to be heterozygous for a
trait if the organism has both a dominant and
recessive allele. - Example Tt is a heterozygous allele combination.
- All hybrids are heterozygous individuals.
18Why Mendel was Important?
- Before Mendel, scientists thought that the traits
of an individual were simply a blend of the
parents traits. - Example If a tall plant and a short plant
reproduced, they would make medium sized plants. - Because of Mendels experiments, traits are
determined by individual, separate alleles
inherited from each parent. - Mendels discovery was not recognized during his
lifetime. - His work was rediscovered in 1900.
- Mendel is known as the Father of Genetics.
19Probability and Genetics
- Mendel carefully counted all of the offspring
from every cross he carried out. - When he crossed two tall hybrid plants, ¾ of the
F2 generation were tall and ¼ were short. - Each time he repeated the cross, he obtained
similar results. - He realized that probability applied to his work.
20Probability and Genetics
- Mendel could say that the probability of
producing a tall plant in the F2 generation was 3
in 4. - The probability of producing a short plant in the
F2 generation was 1 in 4. - Mendel was the first scientist to recognize that
the principles of probability can be used to
predict the results of genetic crosses.
21Punnett Squares
- A Punnett Square is a chart that shows all the
combinations of alleles that can result from a
genetic cross. - Geneticists use these to show all the possible
outcomes of a genetic cross, and to determine the
probability of a particular outcome.
22How to Make a Punnett Square
- Draw a square and divide it into 4 smaller
squares.
23How to Make a Punnett Square
- Place the alleles from one parent along the top
of the Punnett square. - Make sure that only one letter is above each box.
- Place the alleles from the other parent along the
left side of the square. - Make sure that only one letter is beside each box.
T T
t
t
24How to Make a Punnett Square
- Copy the alleles from the top into each box under
them.
T T
T
T
t
T
T
t
25How to Make a Punnett Square
- Now place each letter on the left of the box into
the boxes to the right of them. - When you are finished, you should have two
letters in each box. - You always should write the dominant allele on
the left-hand side. - Tt instead of tT.
T T
Tt
Tt
t
Tt
Tt
t
26How to Make a Punnett Square
- The boxes in the Punnett square represent all the
possible combinations of alleles that the
offspring can inherit. - In this Punnett square, we see the results of
crossing a purebred tall plant with a purebred
short plant. - All of the offspring are hybrid tall plants.
- From this cross, 4 in 4, or 100 will be tall.
T T
Tt
Tt
t
Tt
Tt
t
27Using a Punnett Square
T t
- In a genetic cross, the allele that each parent
will pass on to its offspring is based on
probability. - In the Punnett square to the right, there is a 3
in 4 chance, or 75 chance that the offspring
would inherit the tall trait. - The Punnett square represents the chances each
time a pair reproduces. - This does not mean that if the pair to the right
had 4 offspring, 3 would be tall and 1 would be
short. - It says that each time they reproduce there is a
75 chance for tall plants and 25 chance for
short.
TT
Tt
T
tt
Tt
t
28Phenotypes and Genotypes
- A pheontype is an organisms physical appearance
or visible traits. - Example tall, short, purple flowers, white
flowers, wrinkled seeds, round seeds, black fur,
white fur - A genotype is its genetic makeup or allele
combinations. In other words, the combination of
letters. - Example TT, Tt, tt, BB, Bb, bb, RR, Rr, rr, WW,
Ww, ww
29Codominance
- For all the traits that Mendel studied, one
allele was dominant while the other was
recessive. - This does not happen 100 of the time.
- In codominance, the alleles are not dominant nor
recessive. - As a result, both alleles are expressed in the
offspring.
30Genetic Laws
- The Law of Dominance states that when an
organism has two different alleles for a trait,
the allele that is expressed, overshadowing the
expression of the other allele, is said to be
dominant. The allele whose expression is
overshadowed is said to be recessive.
31Genetics Laws
- The Law of Segregation states that the alleles
for a trait separate when gametes (egg and sperm)
are formed. These allele pairs are then randomly
united at fertilization. Mendel arrived at this
conclusion by performing monohybrid crosses.
These cross-pollination experiments were with pea
plants that differed in one trait, such as pod
color.
32Genetics Laws
- The Law of Independent Assortment states that
alleles for different traits are distributed to
sex cells and offspring independently of one
another. - This means that the inheritance of one trait has
nothing to do with the inheritance of another. - Example Just because a pea plant inherits the
tall trait does not mean that they must also
inherit the trait for having wrinkled seeds.
33Genetic Disorders and Recessive Genes
- Many genetic disorders are caused by recessive
genes. - If an offspring receives two recessive alleles
from the parents, the child inherits the disease. - If a person is heterozygous, he/she will not show
the symptoms. - These people are known as carriers.
34Cystic Fibrosis
- This is a genetic disorder in which the body
produces abnormally thick mucus in the lungs and
intestines. - The mucus fills the lungs and makes it hard to
breathe. - It is caused by a recessive allele on one
chromosome. - It is the result of a mutation in which three
bases are removed from DNA.
35Sickle Cell Disease
- Sickle cell anemia results from a substitution
mutation of the DNA in the sex cells. This has
resulted in a recessive trait. - Sickle cell commonly affects people of African,
Indian, and Mediterranean descent. - It causes the red blood cells to become
sickle-shaped. - This prevents the blood from passing normally
through the capillaries, resulting in oxygen not
being passed on to the tissues.
36Hemophilia
- This is a genetic disorder in which a persons
blood clots very slowly or not at all. - They do not produce one of the proteins needed
for normal blood clotting. - Have a high risk of internal bleeding from small
bumps and bruises. - Caused by a recessive allele on the X chromosome,
making it a sex-linked disorder. - Occurs more often in males than females.
37Heredity and Meiosis
- Sometimes mistakes happen during meiosis, the
production of egg and sperm cells. - This can result in individuals having more or
fewer chromosomes than normal. - Individuals with Downs Syndrome have an extra
copy of chromosome 21. - This results in a variety of physical and/or
mental conditions.
38Sex Chromosomes
- The sex chromosomes carry genes that determine
whether a person is male or female. They also
carry genes that determine other traits. - The sex chromosomes are the only pair that do not
always match. - In females, the chromosomes match. The female
genotype is XX. - In males, the chromosomes do not match. The male
genotype is XY.
39Sex-Linked Genes
- Genes on the X and Y chromosomes are called
sex-linked genes because their alleles are passed
from parent to child on a sex chromosome. - Traits controlled by sex-linked genes are called
sex-linked traits. - One sex-linked trait is red-green colorblindness.
- A person with this trait cannot distinguish
between the colors red and green.
40Pedigrees
- A pedigree is a chart or family tree that
tracks which members of a family have a
particular trait. - Pedigrees include two or more generations.
- Females are represented by circles, while males
are represented by squares. - Those with a trait are shaded, while those that
do not have a trait are left clear. - If the organism is a carrier of a trait, but does
not show the trait, their symbol is only shaded
halfway.
41A Pedigree for Albinism(A condition where the
skin, hair, and eyes lack normal coloring)