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Bio100’15 Medina
Chapter 9
Patterns of Inheritance
Objectives:
1. Using a Punnett square to solve a monohybrid cross, explain Mendel’s Law of Segregation. Provide offspring’s phenotypic
and genotypic ratios.
2. Explain Mendel’s Law of Independent Assortment and identify it in meiosis.
3. Use a test-cross to determine an unknown genotype.
4. Use a family pedigree to determine patterns of genetic inheritance: autosomal recessive, autosomal dominant, X-linked
dominant and X-linked recessive using a specific genetic disorder.
5. Use actual examples to compare and contrast the phenotypic characteristics of the following: incomplete dominance,
codominance, multiple allelism, polygenic inheritance, pleiotropy, effect of environment on genes.
6. Explain how to test for the health of an unborn child.
CONTENT
I.
II.
III.
Heritable Variation and Patterns of Inheritance.
Variations on Mendel’s Laws.
Prenatal Genetic Testing
I.
Heritable Variation and Patterns of Inheritance.
Family resemblance: mother & father contribute to genetic make up. Each human being inherits one
maternal & one paternal set of 23 chromosomes each. Heredity: refers to the greater resemblance of
offspring to parents than to others in the population as a consequence of passing over of traits from parents
to offspring through their genes. Genetics: the study of the mechanisms of heredity. Single-gene traits or
Mendelian inheritance: traits carried in one gene, >9,000 human traits; i.e. cleft chin, free earlobe, widow’s
peak; easiest pattern of inheritance.
Checking:
1. Why do we resemble both of our parents?
2. Define heredity and genetics. What is the simplest pattern of inheritance?
A. Mendel learned about heredity by conducting experiments on garden pea plants.
1) Gregor Mendel, born 1822, lived as a monk in a monastery in the Czech Republic. He was later
called the Father of Genetics for his contribution to find how genetic material is passed on from
parents to offspring. He used mathematics and statistics with his pea plants.
2) Ideal organism to be studied: easy to maintain, easy to breed, reproduces quickly to allow
observation of multiple generations.
3) Numerous traits with 2 variants each: flower color: purple/white; pea shape round/wrinkled; pea
color green/yellow. No intermediates.
4) Established distinct or true-breeding pea plants by repeatedly breeding plants of the same trait:
self-pollination.
5) Mendel applied methodical experimentation and rigorous hypothesis testing.
B. Law of Segregation: during the formation of gametes, the 2 alleles (homologous chromosomes) for
a gene separate from each other during Anaphase I. A gamete (egg/sperm) has only one copy of the
genetic information (23 chromosomes), aka haploid. Monohybrid cross:
1) Mendel cross-bred pure purple flower with pure white flower pea plantsfirst generation all
purplehe then crossbred plants of this first generationsecond generation: 3/4 were purple +
1/4 white; PP, 2Pp, pp. P=purple, p=white. He developed the following hypotheses to explain
this pattern of inheritance: Each parent contributes with a single set of instructions for the trait
(i.e. gene for flower color).
2) Offspring receives 2 copies of the instructions for the trait. Sometimes these instructions are the
same and sometimes they are slightly different (purple-flower vs. white-flower alleles in the gene
for flower color).
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3) The trait observed (phenotype) on the individual depends on the two copies of the gene
(genotype) it inherits from its parents.
a) Homozygous: same 2 alleles for a gene. Example: 2 purple (PP=homozygous dominant)
or 2 white (pp=homozygous dominant)
b) Heterozygous: different alleles for a gene. Example: one purple + one white (Pp).
Perform the following monohybrid crosses using Punnett squares: male (♂), female (♀)
Two pure plants
Two heterozygous parents
One pure plant and one heterozygous parent
Phenotypes:_______________________
_____________________________________
_____________________________
Genotypes:________________________
_____________________________________
_____________________________
C. A test-cross enables us to figure out which alleles an individual carries.
Problem: A zoo keeper wants to know if his brown alligator is a carrier of the gene for albinism (recessive
gene, no pigment production). He is able to do a test-cross by cross-breeding the brown alligator with an
albino alligator.
If brown alligator is BB: Phenotypic ratio of offspring:___________________________________
Genotypic ratio of offspring:____________________________________
If brown alligator is Bb: Phenotypic ratio of offspring:___________________________________
Genotypic ratio of offspring:___________________________________
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D. Mendel’s Law of Independent Assortment: When observing the phenotypes of offspring of a
dihybrid cross (parental varieties based on 2 characters or traits, rather than one), he supported the
hypothesis that each pair of alleles segregates independently of each other during gamete formation.
That means, homologous chromosomes randomly align themselves at Metaphase I and achieve
random or independent separation from each other during Anaphase I.
E. Human disorders controlled by a single gene.
Autosomal Genetic Disorders:
Autosomal
Recessive: Tay-Sachs: Chromosome 15
Dominant: Huntington’s Disease: Chromosome 4
Chromosomes
X-linked Genetic Disorders:
1-22
Recessive: Red-Green colorblindness
Mostly expressed on males because females another X-Chrom.
XY
Dominant: Hypertrichosis (one type)
23
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F. Pedigrees are tools used by geneticists to decipher and predict the inheritance patterns of genes.
It documents a trait of interest across multiple generations of family members. Analyze & label
the following examples: Are both sexes equally affected? Does it skip generations? What
percentage is affected in each generation, ¼ or ½ ?
Checking:
3. Who was Mendel? Why did he use the pea plant as a model?
4. Define homozygous and heterozygous.
5. What were the offspring’s phenotypes & genotypes when he cross-bred pure purple + pure white?
6. What were the offspring’s phenotypes & genotypes when he crossed-bred heterozygous purple offspring with each other?
7. What does Mendel’s Law of Segregation say? Where does this happen in meiosis?
8. What does Mendel’s Law of Independent Assortment say? Where does this happen in meiosis?
9. What is a test-cross?
II.
Variations on Mendel’s Laws
A. Incomplete dominance and codominance: Incomplete dominance heterozygous looks
intermediate between two homozygous. A gene for pigment production is either CR red pigment or
CW no pigment. When homozygous for red CRCR and white CWCW plants are cross bred the result is
all offspring CWCR pink color, which is intermediate between 2 homozygous. We don’t identify them
as dominant or recessive because when they are together neither dominates.
Codominanceheterozygous shows both traits at the same time. Sickle-cell disease is homozygous
recessive, it’s characterized by spindle-shape red blood cells (RBC) with the incapacity to bind
oxygen, therefore producing shortness of breath. The heterozygous has both, normal and sickle cells.
B. Multiple allelism: a single gene has more than 2 alleles. 2 alleles in one individual but more than 2
in a population. Gene that determines blood type can be A, B, O (no proteins on surface of RBC). A
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can donate to A or A/B and receive from A or O; B can donate to B or A/B and receive from B or O.
A/B can only donate to A/B, universal receiver; O universal donor, can only receive from O.
C. Polygenic inheritance: one trait influence by many genes, i.e. height, eye & skin color.
Hint to memorize:
D. Pleiotropy: one gene influences many traits, i.e. Tay-Sachs: caused by a mutation on ONE gene on
chromosome____ multiple symptoms: lipid accumulation on brain cells, mental deficiency,
muscular stiffness, blindness, death in childhood, non-lethal if expressed after 20’s.
Hint to memorize:
E. Sex-linked traits: gene responsible for producing proteins that distinguish red and green wavelength
in the eye is found in the sex chromosome. At least one functioning copy of the gene confers normal
detection of red and green colors. A rare allele produces a non-functioning version of these proteins.
Females get XX and thus get a greater chance to be normal, males get only one X, if the non-the
functioning allele is here, then he cannot produce these proteins at all and becomes red-green colorblinded (7-10% of men, females <1%).
F. Environmental effects, genes interact with the environment to produce physical characteristics and
thus environmental variation influences the expression of genes. i.e. Siamese cats have darker fur
colors on colder areas of their bodies. Genes that produce dark pigments are sensitive to heat, the
colder the greater dark pigment production.
11. What is a Pedigree?
12. Define incomplete dominance, codominance, multiple allelism, polygenic inheritance, pleiotropy, sex-linked traits and provide one
example for each.
13. Is there a relationship between genes and the environment? Provide one example.
III.
Prenatal Genetic Testing
Accuracy of more than 99%, used to identify Down Syndrom & other trysomy-related disorders, Tay-Sachs,
Cystic Fibrosis, Muscular Dystrophy, Spina Bifida (only with amnio). Risks 1% or 1/200: miscarriage,
injury, infection, preterm labor.
A. Amniocentesis: Test of amniotic fluid using a needle and ultrasound; 15th-18th week of pregnancy,
B. Chorionic Villus Sampling (CVS): transcervical or transabdominal sampling of villi; 10th – 12th
week of pregnancy.
http://www.mayoclinic.org/tests-procedures/chorionic-villus-sampling/multimedia/chorionic-villussampling/img-20008547
Textbook problems you may tackle (p. 170): 1-4, 6-8, 11, 12, 16, 17
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Chapter 10: Structure & Function of DNA
1 pt participation
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On a HALF A SHEET OF WHITE PAPER copy Figure 10.8, The Flow of Genetic Information in a
eukaryotic cell, p.178, of your textbook. Please, color beautifully!
Write down the definition for TRANSCRIPTION & TRANSLATION under the terms on your drawing.
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