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Learning Objectives:
1.
2.
3.
4.
5.
Describe Mendelian Inheritance.
Point Out Autosomal Traits and Sex-Linked Traits.
Understand a Punnett Square.
Construct a Pedigree.
Write and Tell Personal Experiences of Individuals with Mendelian Inheritance.
Learning Activities:
Understanding patterns of illness
transmission requires knowledge
MEDIA
of the fundamental principles of
inheritance. Since Gregor Mendel
first noticed the various patterns
of gene segregation for particular
traits in garden peas and was
able to calculate the likelihood of
a trait recurring in later
generations, the inheritance
Click the URL below for further explanation.
URL: https://www.youtube.com/watch?v=Mehz7tCxjSE
patterns of single gene illnesses
are frequently referred to as
Mendelian. A complete family history will be necessary to determine a pattern of
transmission if a family is affected by a disease.
MENDELIAN INHERITANCE IN HUMANS
Mendelian inheritance is the term used to describe the transmission of traits that are
regulated by a single gene with two alleles, one of which may be dominant over the
other. Even though just a few human traits can be determined by a single gene with
two alleles, they are a good place to start when trying to comprehend human heredity.
Whether a trait is governed by genes on the X chromosome or the autosomes will
affect how the trait is inherited.
Autosomal Traits
Genes on one of the 22 pairs of human autosomes are responsible for controlling
autosomal characteristics. Autosomal traits are inherited in the same way regardless
of the sex of the parent or offspring because autosomes are all the chromosomes
other than the X and Y chromosomes.
Figure 1.1 Hitchhiker’s Thumb
Figure 1.2 Cystic Fibrosis
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Sex-linked Traits
Sex-linked traits are those that are regulated by genes on the sex chromosomes. The
majority of sex-related traits are governed by genes on the X chromosome because
of the small size of the Y chromosome. They are known as X-linked characteristics.
Because males only have one X chromosome, single-gene X-linked traits inherit
differently from single-gene autosomal traits. Every male receives an X chromosome
from their mother, and they all pass it on to their daughters but not to their sons.
The Pedigree
A pedigree demonstrates how a trait is passed down within a family from one
generation to the next. A pedigree can provide information such as whether a
characteristic is X-linked, autosomal dominant, or autosomal recessive. Pedigrees
reveal connections and identify people who share a particular trait.
Figure 1.3 Autosomal Dominant Trait Pedigree
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•
•
•
Affected individuals are shown in red, unaffected individuals are shown in blue
Males are shown as squares, females are shown as circles
The top row of a pedigree is the original couple. Two individuals who are connected
by a horizontal line are breeding pairs. The children of the couple are connected to
them by vertical lines.
The next row of the pedigree shows the couple's children, as well as the partners of
the children. And, the third row of the pedigree shows the next generation (the
grandchildren of the couple at the top of the pedigree). Larger pedigrees can have
more rows showing additional generations.
Pedigree Analysis
Click the video on the left or the URL below:
https://www.youtube.com/watch?v=Gd09V2AkZv4
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Figure 1.3 pedigree begins with an affected male and an unaffected female. The
couple's first child (on the left) is an affected male whose partner is an unaffected
female. They produce two children: an affected female and an unaffected male. The
next child is an unaffected male partnered with an unaffected female. Their children
are two unaffected females and an unaffected male. The next child is an unpartnered
affected female. The fourth child is an unpartnered unaffected female. The youngest
child is an affected female partnered with an unaffected male. Their children are an
affected male, followed by an unaffected male, followed by an affected female,
followed by an unaffected female.
Figure 1.1 Is an example of Autosomal Dominant Trait
Hitchhiker’s Thumb: https://www.youtube.com/watch?v=O5TTUgo0dlo
Figure 1.4 Autosomal Recessive Trait Pedigree
In this pedigree, individuals that are half-shaded are heterozygous (and therefore do
not show the trait). In this example, two heterozygous individuals have four children.
Their youngest child is an affected male. This individual mates with an unaffected
female to produce two heterozygous children and two unaffected children. The original
couple's next youngest is a heterozygous female. The original couple's second child
is an unaffected male, who partners with an unaffected female to produce two
unaffected children. The original couple's oldest child is an unaffected female whose
partner is a heterozygous male. Their children are an unaffected female, an unaffected
male, and a heterozygous male.
Figure 1.2 Is an example of Autosomal Recessive Trait
Cystic Fibrosis: https://www.youtube.com/watch?v=DtL-3VIBHTs
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Punnett Square
A chart known as a Punnett square makes it simple to ascertain the predicted ratios
of potential genotypes in the offspring of two parents. Cross-mating is the union of two
parents. The British geneticist Reginald C. Punnett, who created the Punnett square,
is honored by the name. You can see a hypothetical example in Figure 1.5.
Color Blindness is an X – linked
trait.
Inheritance
of
Sex
Chromosomes. Female parents
pass only X chromosomes to
their offspring. Male parents
always
pass
their
X
chromosome to their female
offspring
and
their
Y
chromosome to their male
offspring. Color blindness is an
X-linked recessive trait. Female
parents pass the recessive allele
for the trait to their male
offspring, who pass it to their
female offspring.
Figure 1.5 Punnett Square for Color Blindness
Males only have one allele for any X-linked trait since they have one X chromosome.
An X-linked recessive allele is thus always expressed in males. Females have two
alleles for any X-linked characteristic since they have two X chromosomes. In order
for them to express the recessive trait, they must inherit two copies of the recessive
allele. This explains why females are less likely than males to have X-linked recessive
traits.
Hemophilia, a disease marked by an abnormal blood clotting process, is another
example of a recessive X-linked Mendelian characteristic. Victoria, queen of England
possessed the condition. Two of her five daughters were carriers of the hemophilia
allele, which they acquired from their mother. When they wed members of other
European royal families, notably those from Spain, Germany, and Russia, they
disseminated the allele throughout Europe. It makes sense that hemophilia was once
referred to as "the royal disease."
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Shown below is a pedigree chart (Figure 1.7) of the inheritance of color blindness in a
family. Can you predict the missing phenotype in the chart with the help of the given
Punnett square?
Figure 1.7 Inheritance of Colorblindness in a family
Assessment:
1. Explain why autosomal and X-linked traits have different patterns of inheritance.
2. What is a pedigree, and why is it useful for studying how traits are passed from
one generation to the next?
3. What is a Punnett square, and what does it show?
4. Identify examples of human autosomal and X-linked traits.
5. Explain why fathers always pass their X chromosome down to their daughters.
6. Search videos on YouTube about life stories of individuals who have Mendelian
Inheritance (Color Blindness, Cystic Fibrosis etc.). Write their life story and
share it to the class.
References:
1. (n.d.). 5.13 Mendelian Inheritance. Human Biology. Retrieved November 23,
2022, from https://humanbiology.pressbooks.tru.ca/chapter/5-12-mendelian
inheritance/?fbclid=IwAR2GYgt0425EohwPeVp2HakvVp9JpmS4R0NQ9ghhIL
uYljeJpnoq67Xzk
2. (2022, February 8). 3.11 Mendelian Inheritance. Flexbooks CK-12. Retrieved
November 23, 2022, from https://flexbooks.ck12.org/cbook/ck-12-biologyflexbook-2.0/section/3.11/primary/lesson/mendelian-inheritance-in-humansbio/?fbclid=IwAR3fgH29PdKPpQF1VGt6T67e4jlxXgk4bHqEeGBptEpGTzXW
bY6M_kTFAo
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3. (2022, May 10). MENDELIAN INHERITANCE. National Human Genome
Research
Institute.
Retrieved
November
23,
2022,
from
https://www.genome.gov/genetics-glossary/MendelianInheritance?fbclid=IwAR2g2lTnqyUByqttrB6zDcjy87VBR_87snZs6KGmX1TX
4niPpBMcROSf0Y
4. Wakim, S., & Grewal, M. (2021, December 29). Simple Inheritance. Libre Texts
Biology.
Retrieved
November
23,
2022,
from
https://bio.libretexts.org/Bookshelves/Human_Biology/Book%3A_Human_Biol
ogy_(Wakim_and_Grewal)/08%3A_Inheritance/8.4%3A_Simple_Inheritance?f
bclid=IwAR0BSKw00QEu6wh37pJ6H8JoGOeh2IcBI6dDzBIRFB_LzqgdKvn2
gt23vVw#:~:text=Autosomal%20traits%20are%20controlled%20by,of%20the
%20parent%20or%20offspring
5. Image copyright to Hitchiker's thumb. Human Genetics Myths. (n.d.). Retrieved
November 24, 2022, from https://humangeneticsmyths.weebly.com/hitchikersthumb.html
6. Image Copyright to Mayo Foundation for Medical Education and Research.
(2021, November 23). Cystic fibrosis. Mayo Clinic. Retrieved November 24,
2022,
from
https://www.mayoclinic.org/diseases-conditions/cysticfibrosis/symptoms-causes/syc-20353700
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