Science 9 Module 5

ST.
MARK COLLEGE
Sta. Barbara, Baliuag, Bulacan
Tel. 308 – 9069
Name: ________________________________________
Grade and Section: _____________________________ SCIENCE 9
Module No. 5
Date: ________________
HEREDITY, INHERITANCE AND VARIATION
Review:
A dominant trait is an inherited characteristic that appears in an offspring if it is contributed from

a parent through a dominant allele. Traits, also known as phenotypes, may include features such as
eye color, hair color, immunity or susceptibility to certain diseases and facial features such as dimples
and freckles.
In sexually reproducing species, each individual has two pairs of chromosomes; humans have
23 pairs of chromosomes, and so 46 chromosomes in total. The chromosomes contain thousands
of genes which code for the proteins that express and control all of the biochemical and physical
features of an organism; this set of genes is an organism’s genotype.
Within each chromosome, there are two copies of each gene. Each chromosome carries the
same gene in the same position (called a locus) so that they are paired. However, each locus may
have two different versions of each gene: one received from the mother and one from the father. Each
of the alternative versions of a gene is called an allele. Alleles come in two different
forms: recessive (denoted as a small letter, e.g., a) and dominant (denoted as a capital letter, e.g., A).
If an individual carries the same two alleles for a gene, they are homozygous for that gene
(aa or AA); this is the case whether the alleles are recessive or dominant. If the two alleles are
different, the individual is heterozygous for the gene (Aa).
Assuming Mendelian Genetics, which is a simplified explanatory tool:
● A recessive trait will only be expressed if the offspring has two copies of the recessive allele that
codes for the trait (recessive homozygous, aa).
● A dominant trait will always be expressed in the offspring if the dominant allele is present, even
if there is only one copy of it (heterozygous or dominant homozygous, Aa or AA).
In Grade 8, were introduced to the Mendelian Inheritance and you were able to discover how traits
are transferred from parents to their children. The pattern of inheritance follows laws proposed by
Gregor Mendel. Although variations in heredity was not fully explained by these laws, it paved the way
for other scientists to study other truths in heredity, which could not be explained using the Mendelian
Principles alone. Walter Sutton and Theodore Boveri discovered that inherited traits are in the
chromosomes and that genes are found in the chromosomes.
In this module, you will learn about the Non-Mendelian inheritance, the disorders related to
chromosomes, and the structure of the genetic material – the DNA.
● A gene is a unit of hereditary information.
● An allele is a variant form of a gene. When the copies of a gene differ from each other, they are
known as alleles. A given gene may have multiple different alleles, though only two alleles are
present at the gene’s locus in any individual.
● A genotype is an organism's complete set of heritable genes, or genes that can be passed
down from parents to offspring.
● A phenotype is the observable trait or characteristics of an organism that result from the
interaction of its genotype (total genetic inheritance) with the environment.
● Homozygote, an organism with identical pairs of genes (or alleles) for a specific trait. If both of
the two gametes (sex cells) that fuse during fertilization carry the same form of the gene for a
specific trait, the organism is said to be homozygous for that trait.
● Heterozygous organism or heterozygote, the genes for a specific trait are different.
Genotype and phenotype are two fundamental terms in the science of genetics. The two terms are
often used at the same time to describe the same organism, but there is a difference between
genotype and phenotype:
● An organism's genotype is the set of genes in its DNA responsible for a particular trait.
● An organism's phenotype is the physical expression of those genes.
For example, two mice that look virtually identical could have different genotypes. But if they have
visibly different traits - say, one has white fur and the other has black fur - then they have different
phenotypes.
Non-Mendelian Patterns of Inheritance
Non-Mendelian inheritance is any pattern of inheritance in which traits do not segregate in

accordance with Mendel’s laws. These laws describe the inheritance of traits linked to single genes on
chromosomes in the nucleus. In Mendelian Inheritance, each parent contributes one of two possible
alleles for a trait. If the genotypes of both parents in a genetic cross are known, Mendel’s law can be
used to determine the distribution of phenotypes expected for the population of offspring. There are
several situations in which the proportions of phenotypes observed in the progeny do not match the
predicted values.
● Incomplete Dominance
Incomplete dominance is when a dominant allele (form of a gene) does not

completely mask the effects of a recessive allele, and the organism's resulting physical
appearance shows a blending of both alleles. It is also called semi-dominance or
partial dominance.
An example is seen in the cross-pollination of a red plant and white plant. The dominant red
color is not completely expressed over the recessive white flower. The third phenotype is the
pink color, a blending of red and white colors.
R = red
W = white (can also use R’)
RW or WR = pink can also be RR or R’R
● Co Dominance
Co dominance is a form of inheritance wherein the alleles of a gene pair in a

heterozygote are fully expressed. As a result, the phenotype of the offspring is a combination of
the phenotype of the parents. Thus, the trait is neither dominant nor recessive.
When a cattle with red fur (CR) is crossed with a white fur (CW), the resulting genotype will be
CWCR or CRCW, which has roan fur where both red and white are expressed. This is a result
from the expression of two dominant traits, but they are not blended together, or there is
no combination of traits.
● Multiple Alleles
Multiple alleles occur when there are more than two alleles available for one trait.
For example, in human blood type, there are three alleles with four possible blood types in ABO
blood typing system. The four human blood types are A, B, AB, and O.
Table 1.ABO Blood Types
Blood Type Genotype Phenotype
A A, A/A, O A
B A, B/B, O B
AB A, B AB
O O, O O
The human blood has three alleles – A, B, O – which are multiple alleles. Both A and B
are dominant blood types. When a cross is made between the two, this will result in
codominance, which is another blood type – AB. Type O is a recessive blood type. It is only
expressed when the genotype of the parents are homozygous O.
Assessement:
Identify what is being described in the following statements. Write your answers on the lines.
(2pts)
___________1.A genetic material is the ______.

___________2.It is any pattern of inheritance in which traits do not segregate in accordance with
Mendel’s laws
___________3.He proposed the Mendelian Pattern of Inheritance.
___________4.It is the observable trait or characteristics of an organism that result from the interaction
of its genotype with the environment.
___________5.It is the unit of hereditary information.
___________6.It occurs when there are more than two alleles available for one trait.
___________7.The human blood has three alleles, what are those?
___________8.Genotype is the set of genes in its DNA responsible for a particular trait.
___________9.It is an organism with identical pairs of genes (or alleles) for a specific trait.
___________10.It is an organism's complete set of heritable genes, or genes that can be passed down
from parents to offspring.

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ST.

A dominant trait is an inherited characteristic that appears in an offspring if it is contributed from

Assuming Mendelian Genetics, which is a simplified explanatory tool:

● A gene is a unit of hereditary information.

Non-Mendelian Patterns of Inheritance

Non-Mendelian inheritance is any pattern of inheritance in which traits do not segregate in

Incomplete dominance is when a dominant allele (form of a gene) does not

Co dominance is a form of inheritance wherein the alleles of a gene pair in a

Table 1.ABO Blood Types

Blood Type Genotype Phenotype

_____1.A genetic material is the .

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