13 Dec The purpose of this Biology Discussion is to help each other understand the main concepts presented in the chapters covered this week. In Week of General B
The purpose of this Biology Discussion is to help each other understand the main concepts presented in the chapters covered this week. In Week of General Biology, we will be covering Chapters 8 – 10.
*One of the entries MUST ask a question about a concept/idea presented in the required readings in the textbook from the chapters covered this week. This should be a question pertaining to material that you personally do not understand or need clarification on and should be at least 40-50 words in length. Question topics cannot be claimed, and it is one question topic per student. This will aid in diversifying the discussion. Broad categories are posted already in the discussion board. Post your question under the category to which it best applies. State your question in the subject line of the post. (Do not use generic titles such as week 1, post 1, etc., and try to avoid duplicating the category name.) This will create a list of questions and that everyone will be able to see.
- Student submitted an appropriate post about the material. This includes asking/presenting an original question in a grammatically correct and logical manner.
- Assignment submitted on time and on a different day than other posts.
- Assignment met word count AND Word Count (WC) is stated at the end of the post.
The purpose of this Biology Discussion is to help each other understand the main concepts presented in the chapters covered this week. In Week of General Biology, we will be covering Chapters 8 – 10. *One of the entries MUST ask a question about a concept/idea presented in the required readings in the textbook from the chapters covered this week. This should be a question pertaining to material that you personally do not understand or need clarification on and should be at least 40-50 words in length. Question topics cannot be claimed, and it is one question topic per student. This will aid in diversifying the discussion. Broad categories are posted already in the discussion board. Post your question under the category to which it best applies. State your question in the subject line of the post. (Do not use generic titles such as week 1, post 1, etc., and try to avoid duplicating the category name.) This will create a list of questions and that everyone will be able to see.
· Student submitted an appropriate post about the material. This includes asking/presenting an original question in a grammatically correct and logical manner.
· Assignment submitted on time and on a different day than other posts.
· Assignment met word count AND Word Count (WC) is stated at the end of the post.
,
Chapter 10
Patterns of Inheritance
Essentials of Biology
SEVENTH EDITION
Sylvia S. Mader Michael Windelspecht
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Because learning changes everything.®
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10.1 Mendel’s Laws
Gregor Mendel
Austrian monk
Worked with garden pea plants in 1860s
When he began his work, most acknowledged that both sexes contributed equally to a new individual.
Unable to account for presence of variations among members of a family over generations
Mendel’s model compatible with evolution
Various combinations of traits are tested by the environment.
Combinations that lead to reproductive success are the ones that are passed on.
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Mendel’s Experimental Procedure
Mendel’s experimental procedure:
Used garden pea, Pisum sativa
Easy to cultivate, short generation time
Normally self-pollinates but can be cross-pollinated by hand
Chose true-breeding varieties—offspring were like the parent plants and each other
Kept careful records of large number of experiments
His understanding of mathematical laws of probability helped interpret results.
Particulate theory of inheritance—based on the existence of minute particles (genes)
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Figure 10.1 Mendel Working in His Garden
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Figure 10.2a Garden Pea Anatomy and Traits
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Figure 10.2b Garden Pea Anatomy and Traits
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One-Trait Inheritance
One-trait inheritance:
Original parents called P generation
First-generation offspring F₁ generation
Second-generation offspring F₂ generation
Crossed green pod plants with yellow pod plants
All F₁ are green pods.
Had yellow pods disappeared?
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Figure 10.3 One-Trait Cross
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Punnett Square
Punnett square:
Shows all possible combinations of egg and sperm offspring may inherit
When F₁ allowed to self-pollinate, F₂ were 3/4 green and 1/4 yellow.
F₁ had passed on yellow pods.
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Mendel’s Interpretation
Mendel reasoned
ratio only possible if:
F₁ parents contained two separate copies of each heritable factor
(one dominant and one recessive)
Factors separated when gametes were formed and each gamete carried only one copy of each factor.
Random fusion of all possible gametes occurred at fertilization.
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Mendel’s First Law
Mendel’s first law of inheritance—law of segregation
Cornerstone of his particulate theory of inheritance
The law of segregation states the following:
Each individual has two factors for each trait.
The factors segregate (separate) during the formation of the gametes.
Each gamete contains only one factor from each pair of factors.
Fertilization gives each new individual two factors for each trait.
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One-Trait Testcross 1
One-trait testcross:
To see if the F₁ carries a recessive factor, Mendel crossed his F₁ generation green pod plants with true-breeding, yellow pod plants.
He reasoned that half the offspring would be green and half would be yellow.
His hypothesis that factors segregate when gametes are formed was supported.
Testcross
Used to determine whether or not an individual with the dominant trait has two dominant factors for a particular trait
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One-Trait Testcross 2
One-trait testcross, continued
If a parent with the dominant phenotype has only one dominant factor, the results among the offspring are
If a parent with the dominant phenotype has two dominant factors, all offspring have the dominant phenotype.
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Figure 10.4 One-Trait Testcross
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The Modern Interpretation of Mendel’s Work
Modern Interpretation of Mendel’s Work
Scientists note parallel between Mendel’s particulate factors and chromosomes
Chromosomal theory of inheritance
Chromosomes are carriers of genetic information.
Traits are controlled by discrete genes that occur on homologous pairs of chromosomes at a gene locus.
Each homologue holds one copy of each gene pair.
Meiosis explains Mendel’s law of segregation and why only one gene for each trait is in a gamete.
When fertilization occurs, the resulting offspring again have two genes for each trait, one from each parent.
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Alleles
Alleles—alternative forms of a gene
Dominant allele masks the expression of the recessive allele.
For the most part, an individual’s traits are determined by the alleles inherited.
Alleles occur on homologous chromosomes at a particular location called the gene locus.
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Figure 10.5 Alleles on Homologous Chromosomes
a. Various alleles are located at specific loci.
b. Duplicated chromosomes show that sister chromatids have identical alleles.
Access the text alternative for slide images.
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Genotype Versus Phenotype
Genotype versus phenotype:
Genotype—alleles the individual receives at fertilization
Homozygous—two identical alleles
Homozygous dominant
Homozygous recessive
Heterozygous—two different alleles
Phenotype—physical appearance of the individual
Mostly determined by genotype
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Table 10.1 Genotype Versus Phenotype
| Allele Combination | Genotype | Phenotype |
| A A | Homozygous dominant | Normal pigmentation |
| A a | Heterozygous | Normal pigmentation |
| a a | Homozygous recessive | Albinism |
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Two-Trait Inheritance
Two-trait inheritance:
Mendel crossed tall plants with green pods (TTGG) with short plants with yellow pods (ttgg).
F₁ plants showed both dominant characteristics—tall and green pods.
Two possible results for F₂
If the dominant factors always go into gametes together, F₂ will have only two phenotypes.
Tall plants with green pods
Short plants with yellow pods
If four factors segregate into gametes independently, four phenotypes would result.
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Figure 10.6 Two-Trait Cross by Mendel 1
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Figure 10.6 Two-Trait Cross by Mendel 2
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Mendel’s Second Law—Independent Assortment
Based on the results, Mendel formulated his second law of heredity.
Law of independent assortment
Each pair of factors segregates (assorts) independently of the other pairs.
All possible combinations of factors can occur in the gametes.
When all possible sperm have an opportunity to fertilize all possible eggs, the expected phenotypic results of a two-trait cross are always
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Two-Trait Testcross
Two-trait testcross in fruit fly
Fruit fly Drosophila melanogaster
Used in genetics research
Wild-type fly has long wings and gray body
Some mutants have vestigial wings and ebony bodies.
L= long, l = short, G = gray, g = black
Can’t determine genotype of long-winged gray-bodied fly (L Blank G Blank)
Cross with short-winged black-bodied fly (lowercase llgg)
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Figure 10.7 Two-Trait Testcross
In this example,
ratio
of offspring indicates L blank G Blank fly was L lowercase l G lowercase g (dihybrid).
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Mendel’s Laws and Probability
Mendel’s laws and probability:
Punnett square assumes:
Each gamete contains one allele for each trait
Law of segregation
Collectively the gametes have all possible combinations of alleles
Law of independent assortment
Male and female gametes combine at random.
Use rules of probability to calculate expected phenotype ratios
Rule of multiplication—chance of two (or more) independent events occurring together is the product of their chances of occurring separately
Coin flips—odd of getting tails is ½, odds of getting tails when you flip 2 coins ½ × ½ = ¼
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Figure 10.8 Mendel's Laws and Meiosis
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10.2 Mendel’s Laws Apply to Humans
Pedigree
Chart of a family’s history in regard to a particular genetic trait
Males are squares.
Females are circles.
Shading represents individuals expressing disorder.
Horizontal line between circle and square is a union.
Vertical line down represents children of that union.
Counselor may already know pattern of inheritance and then can predict chance that a child born to a couple would have the abnormal phenotype.
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Pedigrees for Autosomal Disorders
Pedigrees for autosomal disorders
Autosomal recessive disorder
Child can be affected when neither parent is affected.
Heterozygous parents are carriers.
Parents can be tested before having children.
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Figure 10.9 Autosomal Recessive Pedigree
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Autosomal Dominant Disorder
Autosomal dominant disorder:
Child can be unaffected even when parents are heterozygous and therefore affected.
When both parents are unaffected, none of their children will have the condition.
No dominant gene to pass on
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Figure 10.10 Autosomal Dominant Pedigree
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Figure 10.11 Methemoglobinemia
Genetic disorders of interest
Autosomal disorders
Methemoglobinemia—lack enzyme to convert methemoglobin back to hemoglobin
Relatively harmless, bluish-purplish skin
Division of Medical Toxicology, University of Virginia
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Figure 10.12 Cystic Fibrosis
Cystic fibrosis—autosomal recessive disorder
Most common lethal genetic disorder among Caucasians in the United States
Chloride ion channel defect causes abnormally thick mucus.
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Figure 10.13 Alkaptonuria
Alkaptonuria—autosomal recessive disorder
Lack functional homogentisate oxygenase gene
Accumulation of homogentisic acid turns urine black when exposed to air
Biophoto Associates/Science Source
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Figure 10.14 Sickle-Cell Disease
Sickle-cell disease—autosomal recessive disorder
Single base change in globin gene changes one amino acid in hemoglobin
Makes red blood cells sickle-shaped
Leads to poor circulation, anemia, low resistance to infection
Eye of Science/Science Source
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Figure 10.15 Huntington Disease
Huntington disease—autosomal dominant disorder
Progressive degeneration of neurons in brain
Mutation for huntingtin protein
Patients appear normal until middle-aged—usually after having children.
Test for presence of gene
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10.3 Beyond Mendel’s Laws
Incomplete dominance
Heterozygote has intermediate phenotype.
The best examples are in plants. In a cross between a true-breeding, red-flowered plant strain and a white-flowered strain, the offspring have pink flowers. Crossing the pink plants, the offspring’s phenotypic ratio is 1 red-flowered : 2 pink-flowered : 1 white-flowered.
Familial hypercholesterolemia is an example in humans. Persons with one mutated allele have an abnormally high level of cholesterol in the blood, and those with two mutated alleles have a higher level still.
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Figure 10.16 Incomplete Dominance in Plants
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Figure 10.17 Incomplete Dominance in Humans
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Multiple-Allele Traits
Multiple-allele traits:
ABO blood group inheritance has three alleles
antigen on red blood cells
antigen on red blood cells
i = neither A nor B antigen on red blood cells
Each individual has only two of the three alleles
Both
are dominant to i
are codominant—both will
be expressed equally in the heterozygote.
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Figure 10.18 Inheritance of ABO Blood Type
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Polygenic Inheritance
Polygenic inheritance:
Trait is governed by two or more sets of alleles
Each dominant allele has a quantitative effect on phenotype and effects are additive.
Result in continuous variation—bell-shaped curve
Multifactorial traits—polygenic traits subject to environmental effects
Cleft lip, diabetes, schizophrenia, allergies, and cancer
Due to combined action of many genes plus environmental influences
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Figure 10.19 Height in Humans, a Polygenic Trait
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Environmental Influences
Environmental influences:
In response to UV radiation, melanin is produced.
Human production of melanin in skin increases closer to the equator to protect skin from radiation.
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Figure 10.21 Gene Interactions and Eye Color
Multiple pigments are involved in determining eye color.
(red eye): Mediscan/Alamy Stock Photo; (brown eye): stylephotographs/123RF; (blue eye): lightpoet/123RF
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Pleiotropy
Pleiotropy:
Single genes have more than one effect.
Marfan syndrome is due to production of abnormal connective tissue.
Other examples include sickle-cell anemia and porphyria.
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Figure 10.22 Marfan Syndrome, Multiple Effects of a Single Human Gene
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Linkage
Two traits on same chromosome—gene linkage
Two traits on same chromosome do NOT segregate independently
Recombination between linked genes
Linked alleles stay together—heterozygot
