Download Genetic Notes

Heredity & Genetics
Part One: Mendel and the beginnings
of Genetics:
A. What are dominant & Recessive traits?
B. How can you use a Punnett Square to predict the
different Phenotypes and Genotypes of offspring?
C. What is incomplete dominance?
I- Genetics: Similarities and
differences among parents and
offspring. It studies how offspring
inherit characteristics from their
parents and how DNA is involved.
A-Mendel: in 1866, this Austrian monk
studied traits of pea plants.
1. Mendel studied 7 traits one at a time.
Each trait had two forms.
 Height Tall Vs. Short.
2. Mendel grew short and tall pea plants.
3. Mendel let these plants self-pollinate
for several generations to get pure
breed seeds.
 Self-pollinate = Pollination within one plant
4. Mendel plants these seeds and lets
them grow; then he takes the pollen
from the tall pea plants and puts it on
the short pea plants and vice- versa.
 This is Cross-pollination= Take pollen
from one plant and put it on another
plant.
5. The seeds Mendel collects from
these plants are Hybrids.
 Hybrid = combination of two
varieties of organisms.
6. Mendel plants the hybrid seeds to see
if they will grow tall, short, or
medium.
 They all grow tall.
7.
Mendel self-pollinates the hybrid for
several generations, and plants the
seeds from the hybrids. 3:1 ratio some
grew tall some grew short.
8. Mendel concluded that the trait for tallness was
visible and the trait for shortness was hidden.
 Visible trait = dominant, prevents the expression of
recessive trait in organism.
 Hidden trait = recessive.
B-What we learned from Mendel.
1. From his work we know that traits (height)
are determined by Genes.
 Genes = a section of chromosomes that
determines a trait. { eye color, hair color,
etc..}
 Two genes for every trait. Example tallness
in pea plants if pure tall two tall genes, if
short two short genes, if a hybrid one tall
gene and one short gene.
 The different forms of a gene are ALLELES
{recessive & dominant}
C- Using Punnett Squares.
1. Genotype = the genetic make-up of an
organism.
 Homozygous: alleles of a gene are the
same.
 Heterozygous: alleles of a gene are
different; one allele for tall one for
short. Hybrids are heterozygous.
2. Phenotype = What the organism looks
like.
3. Punnett Squares: are used to predict
the genotypes & phenotypes of
offspring.
 Symbols for different alleles of a
gene.
 Dominant is capital and recessive is
lower case letter.
 Height = T for dominant & t for
recessive.
 TT or tt= homozygous
 Tt = heterozygous or hybrid.
4. Offspring receive one allele for each trait from
each parent.
5. Heterozygous pass on either T or t.
D- Incomplete Dominance: Condition in
which the two alleles of a gene are
both dominant.
1-It produces a blended phenotype.
This blended phenotype is the hybrid.
Example: cross a pure red snapdragon with
a pure white snapdragon, and you get a pink
snapdragon. RW is the hybrid note both are
dominant.
Part Two Genes and Traits:
A.In the nuclei of a human cell you find two
sets of chromosomes 23 pairs. This is a
diploid number of chromosomes.
1. One pair of the chromosomes comes
from the father the other set comes
from the mother.
2. Genes are sections of DNA, each
human chromosome may contain
1,000’s of genes.
3. The way chromosomes pair up results
in different genetic make-ups in the
offspring.
B. Meiosis: is the formation of sex
cells or gametes.
1. Eggs are produced in the female sex
organ the ovaries, sperm is produced
in the male sex organ the testes.
2. Meiosis produces sex cells with only
one set of chromosomes these cells
are monoploid.
3. In meiosis the chromosomes separate
twice as do the cells.
4. Steps of Meiosis:
a. 1st everything that occurs in Mitosis
happens with Meiosis. Except the 2nd
time the chromosomes DO NOT
DOUBLE.
b. Sometimes the twin chromosomes that
are similar pair with each other. If they
exchange DNA this is known as
crossing over this increases the
differences in the offspring compared to
the parent.
3. At the end of meiosis we have 4 cells with
a monoploid number of chromosomes.
{HALF THAT OF PARENT CELL}
4. When the egg and sperm cells unite the
two monoploid cells combine their DNA
forming a diploid zygote.
2n Diploid
4n
2n Diploid
n Monoploid
PART THREE
I. How Sex is Inherited: Sex of an
offspring is determined by a pair
of chromosomes.
A. The male body cell has XY
The female body cell has XX
1.During meiosis the eggs of the
female receive an X chromosome.
2.During meiosis the sperm cell of the
male receive either X or Y.
B. Twins:
1. IDENTICAL TWINS: ONE EGG IS
FERTILIZED BY ONE SPERM. THE
ZYGOTE SPLITS. ALWAYS THE
SAME SEX.
Egg
Sperm
Zygote
Same Sex
Always
2. FRATERNAL TWINS: TWO EGGS AND
TWO SPERM CELLS. THEY DO NOT
CONTAIN THE SAME DNA. CAN BE
DIFFERENT SEX.
2 Sperms
2 Egg
2 Zygote
C. X-Linked Traits: are traits
determined by genes on the
x-chromosome.
1. Several diseases are more common
in males then in females.
• Women may not be affected
by the disease, but pass it on
to their sons. These women
are known as carriers.
• X-chromosomes are larger so
they carry more genes. The
ability to see color is on the
x-chromosome.
2) X-chromosomes have genes that tell
whether or not you see color. (Red, Green)
• Women are seldom color-blind because
the gene is a recessive gene on the
X-chromosome and they have
two x-chromosomes.
X’=recessive
gene for colorblindness
3)Hemophilia: is another x-linked
disease it is a blood disorder that
prevents the blood from clotting
properly.
X’ = recessive
trait for
hemophilia
IV.Source of Gene Variations: When
cells divide each DNA molecule in
the cell makes exact copies of itself.
But sometimes the cell makes a
mistake; bases may pair up
incorrectly. The codon are changed
the altered gene is passed on to the
new cell.
A- Mutation is any permanent
change in the DNA of a cell.
1. Mutation increases with the
exposure of DNA to some
chemicals ands radiation that
damage the DNA molecule.
B.Chromosome Mutation: When a
piece of chromosome breaks and
are rearranged or lost. During
meiosis chromosomes mutation
may occur when chromosomes do
not separate evenly.
1. Muscular Dystrophy= Weak muscles.
2. Down Syndrome or Trisomy 21= is
caused as a result of a sex cell having
too many 21st chromosomes. The
fertilized egg, contains three 21st
chromosomes instead of two.
3. Importance of Mutations= are a
source of new traits or new forms of
traits.