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MADANIA (Lower Secondary)
Grade 9 Biology Handout
INHERITANCE
17 / 10 / 2011
Inheritance
• The transfer or transmission of some features from
one generation to the next is called inheritance.
• The characteristics are controlled by genes which
are like coded instruction.
• Genes from one generation are transmitted to the
next in the gametes.
• At fertilization, the gametes fuse to form a zygote
which contains the genetic information from both
parents.
• A new individual grows from the zygote – half of its
genetic information comes from its male parent and
half from its female parent
Gregor Mendel: Father of Modern Genetics
• Mendel was the first scientist to
develop a method for predicting the
outcome of inheritance patterns.
• He performed his work with pea plants,
studying seven traits: plant height, pod
shape, pod color, seed shape, seed
color, flower color, and flower location.
• Pea plants develop individuals that are
homozygous for particular
characteristics. These populations are
known as pure lines.
Mitosis and meiosis
• Mitosis: type of nuclear division that occurs
during growth and asexual reproduction
( somatic cell). The daughter cells are
genetically identical.
• Meiosis: occurs in sex organs to form gametes.
The daughter cells are not genetically identical.
MITOSIS
MEIOSIS
PARENT CELL
(before chromosome replication)
Site of
crossing over
PROPHASE I
Tetrad formed
by synapsis of
homologous
chromosomes
PROPHASE
Duplicated
chromosome
(two sister chromatids)
METAPHASE
ANAPHASE
TELOPHASE
2n
Daughter cells
of mitosis
Chromosome
replication
Chromosome
replication
2n = 4
Chromosomes
align at the
metaphase plate
Tetrads
align at the
metaphase plate
Sister chromatids
separate during
anaphase
Homologous
chromosomes
separate
during
anaphase I;
sister
chromatids
remain together
2n
MEIOSIS I
METAPHASE I
ANAPHASE I
TELOPHASE I
Haploid
n=2
Daughter
cells of
meiosis I
No further
MEIOSIS II
chromosomal
replication; sister
chromatids
separate during
anaphase II
n
n
n
n
Daughter cells of meiosis II
Gregor Mendel: Father of Modern Genetics
• Mendel was the first scientist to
develop a method for predicting the
outcome of inheritance patterns.
• He performed his work with pea plants,
studying seven traits: plant height, pod
shape, pod color, seed shape, seed
color, flower color, and flower location.
• Pea plants develop individuals that are
homozygous for particular
characteristics. These populations are
known as pure lines.
ILLUSTRATION
Mendelian crosses
• In his work, Mendel took pure-line pea plants and
cross-pollinated them with other pure-line pea
plants. He called these plants the parent generation.
• When Mendel crossed pure-line tall plants with
pure-line short plants, he discovered that all the
plants resulting from this cross were tall. He called
this generation the F1 generation (first filial
generation).
• Next, Mendel crossed the offspring of the F1
generation tall plants among themselves to produce
a new generation called the F2 generation (second
filial generation).
Two Innovations of Mendel
1. Developed pure lines
2. Counted his results and kept statistical notes
•
Pure Line (galur murni) - a population that breeds true
for a particular trait
Mendelian crosses
•
To predict the possibility of an individual trait, several
steps are followed:
1. The dominant allele is represented by a capital letter
while the recessive allele by the corresponding
lowercase letter. Homozygous dominant individual (the
genotype is EE); heterozygous individual (the genotype
is Ee); and for a homozygous recessive person (the
genotype is ee).
2. Performing a genetic cross is determining the
genotypes of the parents and the genotype of the
gametes. A heterozygous male and a heterozygous
female to be crossed have the genotypes of Ee and Ee.
During meiosis, the allele pairs separate. A sperm cell
contains either an E or an e, while the egg cell also
contains either an E or an e.
Mendelian crosses
3.
4.
5.
To continue the genetics problem, a
Punnett square is used. A Punnett
square is a boxed figure used to
determine the probability of
genotypes and phenotypes in the
offspring of a genetic cross.
This is done by filling in each square
with the alleles above it and at its left.
Therefore, the ratio of phenotypes is
3 with curly hair to 1 with straight hair
(3:1). The ratio of genotypes is 1:2:1
(1 EE : 2 Ee : 1 ee).
Monohybrid cross
• Involves a study of inheritance patterns for organisms
differing in one traits.
• Using symbols we can predict the cross of tall (DD) and
short (dd) pea plants in the following manner:
Dihybrid Cross Problem Set
• A dihybrid cross involves a study of inheritance patterns
for organisms differing in two traits.
• Mendel invented the dihybrid cross to determine if
different traits of pea plants, such as flower color and
seed shape, were inherited independently.
Terms to know in Mendelian Genetics
1. Alleles: The different forms of a gene. Y and y
are different alleles of the gene that
determines seed color. Alleles occupy the same
locus, or position, on chromosomes.
2. F1 generation
Offspring of a cross between true breeding
plants, homozygous for the trait of interest
3. F2 generation
Offspring of a cross involving the F1
generation.
4. Homozygote - an individual which contains
only one allele at the allelic pair; for example
DD is homozygous dominant and dd is
homozygous recessive; pure lines are
homozygous for the gene of interest
Terms to know in Mendelian Genetics
5. Heterozygote - an individual which contains
one of each member of the gene pair; for
example the Dd heterozygote
6. Incomplete dominance
The flowers of the snapdragon plant can be
red, pink, or white. The genotype RR results
in red flowers and rr results in white flowers.
The heterozygote genotype of Rr results in
pink flowers.
7. Genotype - the specific allelic combination
for a certain gene or set of genes based on
trait.
8. Monohybrid cross.
Cross involving parents differing in only one
trait.
Terms to know in Mendelian Genetics
9. Phenotype
The physical appearance of an organism with
respect to a trait, i.e. yellow (Y) or green (y)
seeds in garden peas. The dominant trait is
normally represented with a capital letter,
and the recessive trait with the same lower
case letter.
10. Recessive trait.
The opposite of dominant. A trait that is
preferentially masked.
11. Dominance - the ability of one allele to
express its phenotype (trait)
Terms to know in Mendelian Genetics
12. Backcross - the cross of an F1 hybrid to one
of the homozygous parents; for pea plant
height the cross would be Dd x DD or Dd x
dd; most often, though a backcross is a cross
to a fully recessive parent
13. Testcross - the cross of any individual to a
homozygous recessive parent; used to
determine if the individual is homozygous
dominant or heterozygous
14. Monohybrid cross - a cross between
parents that differ at a single gene pair
(usually AA x aa)
15. Monohybrid - the offspring of two parents
that are homozygous for alternate alleles of a
gene pair
EXERCISES-1
1.Human blood type is determined by codominant
alleles. There are three different alleles, known as IA, IB,
and i. The IA and IB alleles are co-dominant, and the i
allele is recessive.
The possible human phenotypes for blood group are
type A, type B, type AB, and type O. Type A and B
individuals can be either homozygous (IAIA or IBIB,
respectively), or heterozygous (IAi or IBi, respectively).
A woman with type A blood and a man with type B blood
could potentially have offspring with which of the
following blood types?
A. type A
B. type B
C. type AB
D. type O
E. all of the above
EXERCISES-2
• What are the possible blood types of the offspring of a
cross between individuals that are type AB and type O?
(Hint: blood type O is recessive)
A. AB or O
B. A, B, or O
C. A or B
D. A, B, AB, or O
E. A, B, or AB
EXERCISES-3
• A pea plant is heterozygous for both seed shape and
seed color. S is the allele for the dominant, spherical
shape characteristic; s is the allele for the recessive,
dented shape characteristic. Y is the allele for the
dominant, yellow color characteristic; y is the allele for
the recessive, green color characteristic. What will be
the distribution of these two alleles in this plant's
gametes?
A. 50% of gametes are Sy; 50% of gametes are sY
B. 25% of gametes are SY; 25% of gametes are Sy;
25% of gametes are sY; 25% of gametes are sy.
C. 50% of gametes are sy; 50% of gametes are SY
D. 100% of the gametes are SsYy
E. 50% of gametes are SsYy; 50% of gametes are SSYY.
EXERCISES-4
• Which of the following genetic crosses would be
predicted to give a phenotypic ratio of 9:3:3:1?
A. SSYY x ssyy
B. SsYY x SSYy
C. SsYy x SsYy
D. SSyy x ssYY
EXERCISES-5
• The gametes of a plant of genotype SsYy should have
the genotypes:
A. Ss and Yy
B. SY and sy
C. SY, Sy, sY, and sy
D. Ss, Yy, SY and sy
E. SS, ss, YY, and yy
Monohybrid inheritance
• It concerns the inheritance of a single
characteristic, such as plant height or flower
color
• There are three possible genotypes for plant
height:
1. TT = homozygous tall
2. Tt = heterozygous tall
3. tt = homozygous dwarf
Monohybrid inheritance
• Crossing a homozygous tall plant with a
homozygous dwarf plant