Download Ch. 9 Patterns of Inheritance (Lecture Notes)

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III. Unit 3:
Patterns of Inheritance - Chapter 9
Genetics - The science that is concerned with the study of biological inheritance.
9.1 The science of genetics has ancient roots
Pangenesis - the ancient Greeks believed in the idea that particles governing the inheritance of each trait collect in
eggs and sperm and are passed on to the next generation.
Darwin’s comprehensive theory of heredity and development, according to which all parts of the body give
off gemmules which aggregate in he germ cells; during development, they are sorted out from one another
and give rise to parts similar to those of their origin.
Based on artificial breeding, nineteenty-century observers believed in the “blending” hypothesis, in which traits
from both parents blend in the offspring.
9.2 Experimental genetics began in an abbey garden
Gregor Mendel
At the time Mendel began his study of heredity, a blending theory of inheritance was popular.
Mendel chose to work with the garden pea, Pisum sativum
When these varieties self-pollinated (self-fertilize), they were true-breeding - the offspring were like the parent plants,
and like each other.
Cross-fertilization - The fusion of sperm (pollen) and egg derived from two different individuals. (hybridization or
cross)
Mendel called the original parents the P generation,the first generation offspring the F1 (filial) generation and the
second generation offspring of the F1 plants the F2 generation.
Mendel’s paper, published in an obscure journal in 1866, argued that there are discrete, “inheritable
factors” that retain their individuality when transmitted from generation to generation.
Genes - are the units of heredity within the chromosomes.
9.3 Mendel’s principle of segregation describes inheritance of a single trait
Crosses that involve only one trait are called monohybrid crosses.
In Mendel’s cross of true P organisms, all F1 offspring were like the dominant trait (tall) of the P organisms and
heterozygous (Tt).
In all crosses that he performed, he found a 3:1 ratio in the F2 generation.
A 3:1 ratio among the F2 offspring was possible if:
1. the F1 contained two separate copies (alleles) of each hereditary factor (genes), one of these being dominant and one
being recessive.
2. the factors separated when the gametes were formed and each gamete carried only one copy (alleles) of each factor,
and
3. random fusion of gametes occurred upon fertilization.
In this way, Mendel arrived at the first of his laws of inheritance:
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Mendel’s Law of Segregation. Each organism contains two factors (alleles) for each trait (genes), and the factors
segregate during the formation of gametes so that each gamete contains only one factor from each pair of
factors. When fertilization occurs, the new organism will have two factors for each trait, one from each parent.
Modern Terminology
Alleles - is a pair of genes located at a particular location of homologous chromosomes called the gene locus. In
genetic notation, the alleles are identified by letters. (Tt)
Dominant allele is named because of its ability to mask the expression of another allele and is identified by an
uppercase (Capital) letter. (AA)
Recessive allele is identified with the same letter , but lowercase (small). (aa)
Homozygous is when an organism has two identical alleles (either both dominant or both recessive)
Heterozygous is when an organism has two different alleles (dominant & recessive) at a gene locus.
Genotype and Phenotype
The word genotype refers to the alleles an individual receives at fertilization and is represented by letters.
The word phenotype refers to the physical appearance of the individual. The phenotype is a function of the dominant
alleles. (blue eyes)
Genetics Problems
When solving a genetics problem, it is first necessary to know which characteristic is dominant.
When designating the gametes, it is necessary to keep in mind that although the individual may have two alleles for
each trait, each gamete carries only one allele for each trait.
Punnett Square is a simple method to calculate the results of a cross. Fig 9.3 pg 152 show the results of a cross
between two heterozygous organisms.
The genotype ratio is 1/4 PP : 1/2 Pp : 1/4 pp or 1:2:1
The phenotype ratio is 3 dominant to 1 recessive (Out of 929 F2 offspring, 705 were purple, 224 were white.
The proportions are not exactly 3/4 and 1/4 because mating involves probabilities.)
Mendel observed that each of the seven traits exhibited the same inheritance pattern.
9.4 Homologous chromosomes bear the two alleles for each trait
Chromosomes come in pairs called homologous chromosomes because they carry genes for the same traits.
Although Mendel knew nothing about chromosomes, our knowledge of chromosome arrangements (in
homologous pairs) strongly supports the principle of segregation.
Alleles of a gene reside at the same locus on homologous chromosomes. Fig 9.4 pg 153
9.5 The principle of independent assortment is revealed by tracking two traits at once
Dihybrid Inheritance
Dihybrid crosses are crosses between two organisms that differ in two traits.
Mendel’s Law of Independent Assortment: Members of one pair of factors segregate (assort) independently of
members of another pair of factors. Therefore, all possible combination of factors can occur in the gametes.
All possible combinations of alleles located on different chromosomes occur in the gametes because homologous pairs
of chromosomes separate independently during meiosis. (Only one letter of each kind may appear in a gamete.)
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Mendel always observed a 9:3:3:1 ratio among the F2 generation of a dihybrid cross.
1 = first characteristic
D = dominant
2 = second characteristic
r = recessive
D1 chance = 3/4
D2 chance = 3/4
r1 chance = 1/4
r2 chance = 1/4
chance of D1 D2 phenotype = 3/4 x 3/4 = 9/16
chance of D1 r2 phenotype = 3/4 x 1/4 = 3/16
chance of r1 D2 phenotype = 1/4 x 3/4 = 3/16
chance of r1 r2 phenotype = 1/4 x 1/4 = 1/16
9
3
3
1
See Fig 9.5 pg 154
9.6 Geneticists use the testcross to determine unknown genotypes
The Monohybrid Testcross
In a testcross, an individual with a dominant phenotype is crossed with an individual having the recessive phenotype.
The testcross allows one to determine whether the dominant phenotype is homozygous dominant or
heterozygous dominant.
9.7 Mendel’s principles reflect the rules of probability
Laws of Probability is another method of calculating the expected ratios. The chance or probability that two or more
independent events will occur together is the product of their chances of occurring separately.
(Rule of Multiplication)
The chance of an event that can occur in two or more independent ways is the sum of the individual chances. (Rule of
Addition)
Example
Chance of E (unattached earlobe)
Chance of e (attached earlobe)
Chance of EE = 1/2 x 1/2 = 1/4
Chance of Ee = 1/2 x 1/2 = 1/4
Chance of eE = 1/2 x 1/2 = 1/4
Chance of ee = 1/2 x 1/2 = 1/4
= 1/2
= 1/2
(Rule of Multiplication)
Chance of phenotype with unattached earlobes
Ee + eE + ee = 1/4 + 1/4 + 1/4 = 3/4
(Rule of Addition)
9.8 Mendel’s principles apply to the inheritance of many human traits
Many human traits are thought to be determined by simple dominant-recessive inheritance, and sometimes the
ratio of dominant-to-recessive phenotype exhibits a Mendelian ratio. (Do class count on some traits in
figure 9.8)
The terms dominant and recessive refer only to whether or not a trait is expressed in the heterozygote, not to
whether it is the most common.
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9.9 The relationship of genotype to phenotype is rarely simple.
The inheritance of many traits among all eukaryotes follows the principles that Mendel discovered. However,
most traits are inherited in ways that follow more complex patterns. (incomplete dominance, multiple
alleles at a gene locus, pleiotrophy, and polygenic inheritance.
9.10 Incomplete dominance results in intermediate phenotypes
Incomplete dominance describes the situation where one allele is not completely dominant in the heterozygote; the
heterozygote usually exhibits characteristics intermediate between both homozygous conditions.
Inheritance of alleles that relate to hypercholesterolemia
Normal individuals, HH, have normal amounts of LDL receptors proteins
hh individuals (1 : 106) have no receptors and five times the amount of blood cholesterol
Hh individuals (1 : 500) have half the number o receptors and twice the amount of blood cholesterol.
9.11 Many genes are represented by more than two alleles
The ABO blood groups in humans follow this pattern, in which individuals can have two alleles from a set of three
possible alleles.
The surfaces of red blood cells contain genetically determined blood group antigens (agglutinogens) (carbohydrates)
and the plasma of many persons contains genetically determined antibodies (agglutinins) against the blood
group antigens which they do not have. The ABO and Rh blood grouping systems are based on antigenantibody responses.
ABO blood grouping
Type
Agglutinogen
(Carbohydrate)
A
A
B
B
AB
A, B
O
--
Agglutinins
(antibodies)
b
a
-a, b
Compatible
Donor Blood
A, O
B, O
A, B, AB, O
O
Incompatible
Donor Blood
B, AB
A, AB
-A, B, AB
Type O (unversal donor) has neither carbohydrate and can receive no other type.
Type AB (universal recipient) has both carbohydrates A & B and thus no antibodies and can receive any type of
blood.
9.12 A single gene may affect many phenotypic characteristics
In pleiotropy, the alleles of a single gene locus affect many phenotypic traits.
A relatively uncommon example is the inheritance of an allele that codes for abnormal hemoglobin and, in the
homozygous condition, causes sickle-cell anemia.
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The normal and abnormal alleles are codominant, so heterozygous individuals (carriers) can exhibit some
symptoms, although normally they are healthy.
The incidence of the allele is relatively high in individuals of African descent, because sickle-cell carriers are
somewhat protected from malaria.
9.13 A single trait may be influenced by many genes
In polygenic inheritance, a trait is controlled by several genes, and each allele adds quantitatively to the
phenotype. The variations are continuous - there are many intermediate phenotypes - and the phenotypic
distribution follows a bell-shaped curve.
9.14 Chromosome behavior accounts for Mendel’s principles
We have already seen that the fact that there are homologous pairs of chromosomes accounts for the principle of
segregation.
The fact that there are several sets of homologous pairs of chromosomes accounts for the principle of independent
assortment. Mendel’s seven garden pea traits all sorted independently of each other because the genes
governing each trait are all on separate chromosomes.
9.15 Genes on the same chromosome tend to be inherited together
Linked genes are located close together on the same chromosome.
The inheritance of such genes does not follow the pattern described by the principle of independent assortment
because the two genes are normally inherited together on the adjoining portions of the same chromosome.
The phenotypic ratios of such dihybrid crosses approach that of a monohybrid cross (3:1), rather than the typical
pattern of the dihybrid cross. (9:3:3:1).
9.16 Crossing over produces new combinations of alleles
Cross over recombines linked genes into assortments of alleles not found in parents. Early examples of
recombination were demonstrated in fruit flies by embryologist T. H. Morgan and colleagues in the early
1900s.
9.17 Geneticists use crossover data to map genes
A.H. Sturtevant, one of Morgan’s colleagues, developed a technique of using crossover data to map the locations
of genes on chromosomes on which they were linked. Sturtevant assumed that the rate of recombination is
proportional to the distance apart two genes are on a a chromosome.
The crossing-over frequency (equivalent tot he recombinant phenotype frequency) in crosses involving linked
genes indicates the distance between genes on the chromosomes. (inc. frequency --> inc. dist. btw. loci
(Morgan Units)
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Example
% crossing over: AB 1%, AC 29%, AE 32%, CE 3%, BC 30%, BE 33%, BD 57%, CD 27%
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9.18 Chromosomes determine sex in many species
In humans, XX individuals are female, and XY are male
In other species, other patterns of sex chromosomes exist.
In some species, sex is determined by chromosome number rather that chromosome type.
(XX female, XO (one X) male)
In some invertebrates (ants & bees) diploid individuals are female and haploid are male.
9.19 Sex-linked genes exhibit a unique pattern of inheritance
Sex chromosomes contain genes specifying sex and other genes for traits unrelated to sex. Because of linkage and
location, the inheritance of these traits follows peculiar patterns. In humans, most (known) sex-linked
traits result from genes on the X chromosome.
Some X-linked genes can control traits that have nothing to do with sex characteristics. Males have only one copy
of these genes, and if they inherit a recessive allele, it will be expressed. (Hemophilia, Color Blindness)