SMP SAINS MIFTAHUL HUDA

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Class :

Learning outcomes:
➢ Students are able to explain genes and chromosomes.
➢ Students are able to explain monohybrid and dihybrid crosses.
➢ Students are able to explain about hereditary diseases or congenital abnormalities.

Human heredity is the inheritance of traits from parents to their offspring. The
inheritance of traits from parents to their offspring is studied in a branch of
biology called genetics. Research on the pattern of hereditary traits was
conducted in the nineteenth century by an Austrian monk named Gregor Johann
Mendel (1822-1884).

• After conducting research using snow peas (Pisum sativum) in

1865, the principle put forward by Mendel is known as
Mendel's Law. Because of his services, Mendel is known as
the Father of Genetics.
• In his research, Mendel used pea plants because pea plants
have contrasting characteristics. For example, tall stems and
short stems; can naturally self-pollinate; easy to cross-
pollinate; quickly produce offspring.

• In genetics, genes and chromosomes are closely related. All offspring resulting from generative
reproduction have varying traits that are the result of a mixture of both parents
• Genes: Genes are the hereditary units of a living organism, and are stored in certain positions on
chromosomes. It is estimated that there are 50,000-100,000 genes in humans, found on 23 pairs of
chromosomes.
• Chromosome: a structure that has a thread-like shape in the nucleus of human cells that stores genetic
information.

Chromosomes
• Chromosomes come from two words, namely chroma (color) and soma
(body). This term arises because this part will be clearly visible under a
microscope when given a dye. Chromosomes are located in the nucleus
(cell nucleus). Each type of organism has a different number of
chromosomes.
• Cromosomes in every cell nucleus are always found in pairs. Each
chromosome has its own pair or homolog which are called diploid (2n).
With each of these diploid chromosomes pairs, half comes from the
male parent and the other half comes from the famale parent. Therefore,
both male and female sex cells carry a set of chromosomes, known as
haploid (n). If mating occurs a zygote is produced. The zigote will have
two sets of chromosomes or become diploid (2n) again.
• Based on their function, chromosomes are divided into body
chromosomes and sex chromosomes. Body chromosomes or autosomes
are chromosomes that do not determine sex, numbering 2n-2, because n
is the total number of chromosomes, while autosomes have diploid properties and (-2) is the

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nulisomi chromosomes, when cell loses one pair of chromosomes. In humans, the number of
autosomes in each body cell is 44 (22 pairs). Sex chromosomes or gonosomes are a pair
chromosomes that determine sex.
• The sex chromosomes in women are XX, while the sex chromosomes in men are XY. Thus, the
writing of chromosomes in human body cells: 22 AA + XX or 46, XX (female) and 22 AA + XY or
46, XY (male), in human sperm: 22 A + X or 22 A + Y, and in human ovum (egg cells): 22 A + X.

Chromosomes Structure • Centromere: in the middle of the chromosome

where the chromosome arms are attached. and is an
area that does not contain gene codes (genomes).
• Kinetochore: the part of the centromere where the
spindle threads are attached during cell division.
• Telomere: the end of the chromosome
• Matrix: chromosome fluid
• Chromonema: chromosomes that contain genomes
or gene codes that contain chromomeres
• Chromomeres: beads where loci containing gene
codes are located.
• Satellite: the end of the chromosome is round.
Term in Derivation

Before studying the principles of genetics, it is necessary to first understand several terms as follows:
a. Parental (P), namely the parents (male and female) who mate/cross. Parental is also called the
parent/elder.
b. Filial (F), namely the individual resulting from the cross, also called the offspring/descendant. The
first offspring is given the symbol F, the second offspring is given the symbol F, and so on.
c. Gametes (G), sex cells (in a crossover diagram gameter indicate the allelic variation that can be
inherited by the parental to the fillial).
d. Dominant gene, namely the gene that is able to cover the characteristics of other genes with same
allele.
e. Recessive gene, namely the gene that is covered by the characteristics of other genes with same
allele.
f. Intermediate/codominant gene, namely genes that do not defeat each other or have the same
strong influence.
g. Homozygous, which is a pair of genes that have the same alleles, for example AA or mm.
h. Heterozygous, which is a pair of genes that have different alleles, for example Aa or Mm.
i. Hybrid, which is the result of a cross or the result of a marriage between two individuals that have
different traits. A cross with one different trait is called a monohybrid cross, a cross with two
different traits is called a dihybrid cross, a cross with three different traits is called a trihybrid cross,
and so on.
Genotype and Fenotype

Example :
Genotype is a set of genes determining an
individual’s characteristic. The genotype
causes the emergence of characteristics in
the phenotype.
Phenotype is a characteristic which can
be observed by the senses. It exists
because of interactions between genes and
the environment. It is determined by the
genotype and environmental factor.

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Dominant, Recessive, and Intermediate

• A dominant characteristic is one which always appears or dominants other characteristics.

• A recessive characteristic is one which does not appear or is dominated. For instance, when peas
with round and wrinkled seeds are crossed, the result is a round seed. Consenquently, in that cross,
the pea with the round seed is dominant and the pea with wrinkled seed is recessive.
• An intermediate characteristic is an in between characteristic which occurs because thre are no
dominan genes. For instance, if a plant with red flowers is crossed with a plant with white flowers,
they will result in plant with pink flowers.
Trait 1-2 capital letters
Trait 2-2 capital letters (dominant)
(recessive)

Heterozygous-Mix
(intermediate)

Cross diagram between round and A cross diagram between red and white flowers
wrinkled seed peas which produces in pink flowers. It indicates an
intermediate characteristic

Monohybrid Cross P1 : Red (MM) >< White (mm)

G1 : M m
Monohybrid cross is a cross between two individuals with M m
one different trait. Monohybrid crosses are divided into two types,
namely:
F1 : Mm (Pink 100%)
• Dominant monohybrid cross is a cross between two similar A cross between F1 and F1, so
individuals that observed to one different trait with dominant P2 : Pink (Mm) >< Pink (Mm)
genes. Dominant traits can be seen easily, namely traits that G: M m
appear more in offspring than other traits of the same allele. M m
• Intermediate monohybrid cross is a cross between two similar F2 :
individuals that focus on one different trait with intermediate Gametes M m
genes. M MM Mm
m Mm mm

Dihybrid Cross • Dihybrid crosses are crosses between two

individuals of the same type that involve two
P1 : Yellow Round (BBKK) ><Green Wrinkles (bbkk) different traits, for example a cross between a pea
G1 : B b plant with round and green seeds and a pea plant
K k
with wrinkled and yellow seeds; short-lived and
few-grained rice with long-lived and many-grained
F1 : BbKk (Yellow Round 100%) rice.
A cross between F1 and F1, so
P2 : Yellow Round (BbKk) >< Yellow Round (BbKk)
• Mendel also studied dihybrid crosses in peas.
G: Bb Bb Mendel crossed round and yellow-seeded peas with
Kk Kk wrinkled and green-seeded pea plants. It turned out
F2 :
that all of the F₁ had round and yellow seeds. This
Gametes BK Bk bK bk means that round seeds and yellow color are
BK BBKK BBKk BbKK BbKk dominant traits. Furthermore, all F plants were
Bk BBKk BBkk BbKk Bbkk allowed to self-pollinate. It turned out that in F, 315
plants with round and yellow seeds, 108 plants with
bK BbKK BbKk bbKK bbKk
round and green seeds, 106 plants with wrinkled
bk BbKk Bbkk bbKk bbkk
and yellow seeds, and 32 plants with wrinkled and
green seeds were produced.
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Hereditary Disease or Congenital Abnormalities

Hereditary diseases or congenital defects can be divided into two types, namely :
1. Diseases linked to autosomes (body chromosomes)
a. Albinism is a disorder caused by a recessive gene found in the autosomes. Characteristics: very
sensitive vision, especially to very high light intensity; has weakness in the eye tissue which
among other things results in low eye focus ability; abnormal skin pigmentation.
b. Sickle cell anemia is a disorder caused by a recessive gene and in a homozygous recessive state
causes the sufferer's erythrocytes (red blood cells) to be crescent-shaped. As a result, their ability
to transport oxygen to body tissues is reduced.
c. Syndactyly (sufferers have fingers or toes that are attached to each other).
d. Polydactyly (sufferers have more than five fingers or toes).
e. Brachydactyly (sufferers have short fingers or toes).
2. Diseases linked to gonosomes (sex chromosomes)
a. Hemophilia is a blood disorder, namely the sufferer's blood is difficult to clot. This disease is
caused by a recessive gene linked to the X chromosome and can cause death if in a homozygous
recessive state. Because it is linked to the X chromosome, there are no male carriers of hemophilia
and no female hemophilia (because it is lethal).
b. Color blindness is caused by a recessive gene linked to the X chromosome. Color blindness is
divided into two, namely partial color blindness (partial) and total color blindness. People with
partial color blindness cannot distinguish certain colors, such as red or green, while people with
total color blindness can only distinguish black and white.

Albinism Hemophilia Color blindness

Polydactyly Brachydactyly
Syndactyly

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