This document discusses Mendel's laws of inheritance, sex linkage, and recombination. It provides background on Gregor Mendel and his experiments with pea plants which established the basic principles of genetics. It defines key genetic terms like dominant/recessive, genotype/phenotype, homozygous/heterozygous. It explains Mendel's three laws of inheritance and how Punnett squares can show potential offspring from a genetic cross. It also discusses sex linkage, where genes on the same chromosome tend to be inherited together, and recombination, where genes can become separated during meiosis. The document provides self-assessment questions and examples to practice genetic crosses and inheritance patterns.

1. MENDEL’S LAWS OF INHERITANCE
SEX LINKAGE AND RECOMBINATION
GENERAL BIOLOGY 2

2. COURSE
DESCRIPTION
◼ This subject is designed to enhance the
understanding of the principles and
concepts in the study of biology, particularly
heredity and variation, and the diversity of
living organisms, their structure, function,
and evolution.

3. CONTENT
STANDARD:
MENDEL’S LAWS
OF INHERITANCE

4. PERFORMANCE STANDARD:
Make a Pedigree Analysis in the
Learner’s Family using a simple
genetic trait.
Make a research paper/case
study/poster on genetic diseases.

5. LEARNING
COMPETENCY:
◼Explain the different Mendel’s Laws
of Inheritance.
◼Explain sex linkage and
recombination.

6. GREGOR JOHANN
MENDEL
◼ 1822-1884
◼ Austrian Monk
◼ Father of Modern Genetics
◼ Experimented with pea plants
◼ He thought that “heritable factors” (genes)
retained their individuality generation after
generation
◼ Principles of genetics were developed in the
mid 19th century
◼ Experimented with pea plants, by crossing
various strains and observing the
characteristics of their offspring

7. DOMINANT
AND
RECESSIVE
◼A dominant allele is represented by a
CAPITAL letter. It is always
expressed when present (BB or Bb).
◼A recessive allele is represented by
lower case letter. It is only expressed
when an individual has 2, one from
the mother and one from the father
(bb).

8. GENOTYPE
AND
PHENOTYPE
◼A genotype is the genetic make-up of
an individual expressed in letters (BB,
Bb, bb)
◼A phenotype is the physical
appearance of an individual,
determined by his or her genotype
(black, brown, short, tall, etc.)

9. HOMOZYGOUS
AND
HETEROZYGOUS
◼Homozygous – when both alleles of a
genotype are the same (either both
dominant, BB, or both recessive, bb)
◼Heterozygous – when one allele is
dominant and one is recessive (Bb
only)

10. PUNNETT SQUARES
◼ Punnett squares are
used to show the
mating of two
parents and the
possible offspring
they can produce.

11. MENDEL’S LAWS
OF INHERITANCE
◼Law of Segregation
◼Law of Independent Assortment
◼Law of Dominance
- Complete Dominance
- Incomplete Dominance
- Codominance

12. SEX LINKAGE
AND
RECOMBINATION
◼Sex linkage refers to the association
and co-inheritance of two DNA
segments because they reside close
together on the same chromosome.
◼Recombination is the process by
which they become separated during
crossing over, which occurs during
meiosis.

13. PEDIGREE
ANALYSIS
◼ Pedigree charts
are diagrams that
show the
phenotypes
and/or genotypes
for a particular
organism and its
ancestors.

15. SELF-
ASSESSMENT
QUESTION 1:
WHAT TRAITS DO
YOU THINK YOU
INHERITED FROM
YOUR PARENTS?

16. SELF-
ASSESSMENT
QUESTION 2:
WHY IS IT
IMPORTANT TO
LEARN ABOUT
GREGOR
MENDEL’S LAWS
OF INHERITANCE?

17. SELF-
ASSESSMENT
QUESTION 3:
HOW IS IT
POSSIBLE FOR
PHYSICALLY
LINKED GENES TO
NONETHELESS
SEGREGATE
INDEPENDENTLY?

18. SELF-
ASSESSMENT
QUESTION 4:
IS IT POSSIBLE TO
TRACE A TRAIT
OR GENE THAT
RUNS IN YOUR
FAMILY THROUGH
PEDIGREE
CHARTS?
EXPLAIN YOUR
ANSWER.

19. SELF-
ASSESSMENT
QUESTION 5:
WHY DOES SEX
LINKAGE GO
HAND IN HAND
WITH
RECOMBINATION
?

20. LET’S PRACTICE!
◼Situation 1. Rabbits have two
visible traits. Fur color (black or
white), and eye color (green or
red). The black fur allele (B) is
dominant over the white (b), while
the green eye allele (G) is
dominant over red (g). Construct a
Punnett square to give the
expected genotypes and
phenotypes of the cross.

22. LET’S PRACTICE!
◼ Situation 2. The gene for seed color in
pea plants exists in two forms. There is
one form or allele for yellow seed color
(Y) and another for seed color (y). When
the alleles of a pair are different
(heterozygous), the dominant allele trait
is expressed, and the recessive allele
trait is masked. Seeds with genotypes
of YY or Yy are yellow, while seeds that
are yy are green. Construct a Punnett
Square to give the expected genotypes
and phenotypes of the cross.

25. LET’S PRACTICE!
◼ Situation 3. Let us take an example of
tall and dwarf mango trees. When pure
line tall (TT) trees were crossed with
pure line dwarf (tt) trees, offspring were
all heterozygous tall (Tt). Hence, the
allele tall (T) is dominant over allele
dwarf (t). Construct a Punnett Square
showing the cross of the tall and dwarf
mango trees.

27. LET’S PRACTICE!
◼ A man with hemophilia (a recessive, sex-linked
trait condition) has a daughter of normal
phenotype. She marries a man who is normal for
the trait.
◼ Show the cross for each of the following:
◼ What is the probability that a daughter of this
mating will be a hemophiliac?
◼ That a son will be a hemophiliac?
◼ If the couple has four sons, what is the
probability that all four will be born with
hemophilia?

30. LET’S TRY!
◼ 1. Which of the following statements is true
concerning biological inheritance?
◼ A. Each human somatic cell contains one of
each type of chromosome.
◼ B. When sex cells are produced, paired
homologous chromosomes separate so
that each gamete contains only one of the
pair of alleles for each trait.
◼ C. Brothers and sisters frequently have
exactly the same combination of
chromosomes.
◼ D. The offspring are genetically identical to
their parents.

31. LET’S TRY!
◼ 2. Cystic fibrosis is a hereditary disease that affects
the lungs and digestive system. To have a cystic
fibrosis, one must inherit two copies of the CFTR
gene that contain mutations – one copy from each
parent. That means that each parent must either
have cystic fibrosis or be a carrier of a CFTR gene
mutation. What is the probability of two cystic
fibrosis carriers having a child with cystic fibrosis?
◼ 50%
◼ 0%
◼ 25%
◼ 100%

32. LET’S TRY!
◼ 3. Can a male be a carrier for hemophilia?
◼ A. Yes, he can be a carrier because he has
two X chromosomes.
◼ B. Yes, he can be carrier because he has
both and X and Y chromosome.
◼ C. No, he cannot be carrier because he
has only one Y chromosome.
◼ D. No, he cannot be a carrier because he
has only one X chromosome.

33. LET’S TRY!
◼ 4. The X-linked recessive trait of color-
blindness is present in 5% of males. If a
mother who is a carrier and father who is
unaffected plan to have 2 children, what is
the probability the children will both be male
and color-blind?
◼ 50%
◼ <1%
◼ 25%
◼ 6.25%

34. LET’S TRY!
◼ 5. Which of the following statements is
TRUE about the crossing-over of parts of
the chromosomes?
◼ It has no effect on genetic linkage.
◼ It usually decreases the number of genetic
combinations in a population.
◼ It can increase the number of genetic
combinations in a population.
◼ It remains as it is.

35. LET’S TRY!
◼ 6. Some people are unable to see red and green colors. This
condition, colorblindness, is a recessive trait carried
on the X chromosome, Xb. The following pedigree shows a family in
which some individuals are affected by colorblindness.
◼ Which of the following describes the possibility of this son being
colorblind?
◼ Zero percent, because the father is not colorblind.
◼ Twenty-five percent, because in the Punnett square, only one box
out of four shows an affected male.
◼ Fifty percent, because only one of the two males in the Punnett
square has the genotype for being affected.
◼ One hundred percent, because the mother will pass the
colorblindness trait to all offspring through her X-chromosome.

36. LET’S TRY!
◼7. Which of the following is TRUE
about sex-linked traits?
◼They are common in males.
◼They are all recessive.
◼They are all dominant.
◼They are more common in
females.

37. LET’S TRY!
◼8. A purebred tall plant is crossed
with purebred short plant. All the F1
offspring are tall. Which of the
following is TRUE about the allele for
tallness?
◼It is recessive.
◼It is dominant.
◼It is homozygous.
◼It is heterozygous.

38. LET’S TRY!
◼ 9. Why are sex-linked genes expressed
differently in different cells of a female?
◼ Sex-linked genes are expressed differently in
males than in females.
◼ Different cells inherit different genes.
◼ One of the two X chromosomes is inactivated
at random in each cell.
◼ Males have no functioning X chromosome
after it is inactivated.

39. LET’S TRY!
◼10. Hemophilia is a recessive x-
linked disorder. Which genotype
represents a female who is a carrier
for hemophilia?
◼ A. XHXh
◼ B. XHXH
◼ C. XhXh
◼ D. XhY