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Genetics
Chapter 10
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Genetics
Genetics is the study of inheritance
Genetics can predict how genes may be passed on to future generations
Requires an understanding of – How genes are organized on chromosomes– How chromosomes are passed on during
meiosis
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A Gene
Gene - a segement of DNA that has the necessary information to code for a protein and regulate its expression– contains the promoter, protein coding sequence,
termination sequence Genes are on a chromosome Related to a characteristic of an organism
– proteins affect the characteristics of an organism Eye color, curly hair, sickle-cell anemia Flower color Pea shape
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Alleles
Alternate versions of the same gene
Example 1: gene for earlobe shape– There are two different alleles for this gene.
Attached earlobe Free earlobe
Example 2: gene for hair texture curly hair strait hair
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What is an allele?
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Alleles
Different alleles code for different forms of the same protein.
– The different forms of the protein function differently and result in different characteristics
Wild-type alleles - those forms of the gene that are most commonly observed
Mutant alleles/alternative alleles - a new or less common form of the gene
Example 3: Mutant allele - sickle-cell hemoglobin wild-type allele - normal, healthy hemoglobin
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Genome
The sum total of an organism’s genes– The human genome is spread over 46 chromosomes
Haploid genome (n)– one copy of each gene– only one allele of each gene is present
Diploid genome (2n)– two copies of every gene– the copies may not be identical, so one individual
could have two different alleles– sexually reproducing organisms - one allele from
each parent
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Genomes and Meiosis
Meiosis produces haploid sex cells (gametes)– Sex cells are sperm and egg
When haploid egg joins with haploid sperm (fertilization), a diploid zygote results– fertilization restores the diploid (2n) genome
The zygote receives half of its genome from the sperm and half of its genome from the egg– has a unique set of genes, different from the
parents (recall how meiosis shuffles the genes)
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Chromosomes and Chromosome Pairs
Chromosomes come in pairs Humans have 46 chromosomes arranged in 23
pairs– one chromosome from each pair comes from the
mother, the other chromosome of the pair comes from the father
Homologous chromosomes - a matched pair of chromosomes – homologous chromosomes contain the same genes – may contain different alleles of the same gene
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Homologous Chromosomes
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Human Karyotype
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Fundamentals of Genetics
Three questions allow us to predict how a trait will be inherited:
1. What alleles do the parents have for that trait?
2. What alleles will be present in the gametes that the parents produce?
3. What is the likelihood that gametes with specific combinations of alleles will be fertilized?
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Phenotype vs. Genotype
Genotype - describes the combination of alleles present in the organism’s cells– what genes does the organism have
Phenotype - describes the organism’s appearance– what does the organism look like– what genes are actually expressed
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Example: Earlobe Shape
Phenotypes: free or attached Alleles: E (free) or e (attached) Genotypes:
– EE (two alleles for free earlobes) Earlobes will be free
– ee (two alleles for attached earlobes) Earlobes will be attached
– Ee (one allele for free and one allele for attached) Earlobes will be free
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Example: Earlobe Shape
Because only the free earlobe (E) allele is observed with the Ee genotype, the free earlobe (E) is said to be the dominant allele– E is expressed over e
The attached earlobe allele is said to be the recessive allele
– Masked by the dominant allele when present together– only expressed when two copies of the recessive allele are
present
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Homozygous vs. Heterozygous Genotypes
Homozygous Genotype– Organism has two copies of the same allele
genotype EE is homozygous dominant - phenotype? genotype ee is homozygous recessive - phenotype?
Heterozygous Genotype– Organisms has two different alleles
genotype Ee is heterozygous - phenotype?
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Predicting Genotype from Phenotype
What are the possible genotypes for an individual with free earlobes?
What are the possible genotypes for an individual with attached earlobes?
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Predicting Gametes from Meiosis
The Law of Segregation:– Alleles separate during meiosis and each gamete
will receive one alleleAn EE individual can only make gametes that have allele EAn ee individual can only make gametes that have allele eAn Ee individual can make gametes that have either E or e, but never both
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Predicting Offspring from Fertilization
Fertilization - the process of two haploid sex cells joining to form a diploid zygote– The genotype of the offspring will be determined by
the alleles carried by the gametes
A genetic cross is a planned mating between two organisms– The outcome of a given cross is predicted by a
Punnett Square
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Punnett Square
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Predicting Offspring from Fertilization
Monohybrid cross - single-factor crosses track the inheritance of one trait– example: earlobes E, e
Dihybrid cross - double-factor crosses track the inheritance of two traits– example: earlobes E, e and hair color H, h
When using Punnett Squares:– determine the types of gametes each parent can produce– plug and chug in the Punnett squares– observe the genotypes and phenotypes to answer
questions
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Probability vs. Possibility
Probability is the mathematical chance that an event will happen.– Expressed as a percent, or a fraction
Probability = the # of events that can produce a given outcome/the total # of possible outcomes.
The probability of two or more events occurring simultaneously is the product of their individual probabilities.
Possibility states that an event can happen; probability states how likely the event is to happen.
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The First Geneticist: Gregor Mendel
Mendel was a monk who was the first to describe the basic patterns of inheritance.– Studied inheritance in garden pea plants– Studied several different phenotypes– Identified the concepts of dominance and
recessiveness– Didn’t know about genes or chromosomes– Identified patterns by mathematical analysis of
the data
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Mendel’s Experiment
Parental (P) generation– A pure-breeding purple-flowered plant mated with a pure-
breeding white-flowered plant.– CC x cc
First filial generation (F1)– All offspring had purple flowers (Cc).– They were allowed to self-pollinate.– Cc x Cc
Second filial generation (F2)– ¾ of the offspring were purple– ¼ of the offspring were white– 3:1 ratio, purple: white
Mendel saw this pattern with any of the traits he studied.
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Dominant and Recessive Traits in Pea Plants
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Mendel’s Conclusions
Organisms have two pieces of genetic information for each trait.– We know these as alleles
The Law of Dominance– Some alleles mask other alleles
Gametes fertilize randomly
The Law of Segregation– Alleles separate into gametes during meiosis.
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Solving Genetics Problems: Single-Factor Crosses
The pod color of some pea plants is inherited so that green pods are dominant to yellow pods
A pea plant that is heterozygous for green pods is crossed to a pea plant that produces yellow pods
What proportion of the offspring will have green pods?
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Step I: Make a Gene Key
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Step 2: Identify Information in the Problem
A green plant is crossed with a yellow plant. The green pod plant is heterozygous.
– Gg The yellow pod plant is homozygous.
– gg The cross is Gg x gg.
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Step 3: Determine Possible Gametes from Each Parent
Heterozygous green pod parent (Gg)– Could make gametes with G or g
Homozygous yellow pod parent (gg)– Could make gametes with g
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Step 4: Create a Punnett Square
Put the gametes from one parent on one side.
Put the gametes from the other parent on the other side.
Simulate random fertilization by crossing the possible gametes.– This will determine offspring phenotypes.
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Step 5: Determine Offspring Phenotypes and Calculate Probability
Use the gene key to determine the phenotype of the offspring you predicted.
Revisit the question to calculate the answer to the question.– What proportion of offspring will produce green
pods? The answer is 50%.
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Cross #2: PKU
The normal condition is to convert phenylalanine to tyrosine. It is dominant over the condition for PKU.
If both parents are heterozygous for PKU, what is the probability that they will have
– A child that is normal? – A child with PKU?
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Solution Pathway
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Dihybrid Crosses
Dihybrid crosses track the inheritance of two traits
Mendel used dihybrid crosses to identify the law of independent assortment– States that alleles of one character separate
independently of alleles of another character– Alleles on one chromosome are assorted
independently of those on another chromosome– Only true when the genes for the two characters
are on different chromosomes
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Solving Double-factor Crosses
When solving a double-factor cross, you must obey the law of segregation and the law of independent assortment.– Each gamete must receive only one copy of
each gene.– All combinations of alleles for both traits must be
considered. Consider an individual whose genotype is
AaBb.– Gametes could receive AB, Ab, aB or ab.
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A Sample Double-factor Cross
In humans the allele for free earlobes is dominant over the allele for attached earlobes.
The allele for dark hair dominates the allele for light hair.
If both parents are heterozygous for earlobe shape and hair color, what types of offspring can they produce, and what is the probability for each type?
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Solving the Double-factor Cross
Start by creating a gene key for each gene.
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Solving the Double-factor Cross
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Solving the Double-factor Cross
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Modified Mendelian Patterns
Some alleles have consistent dominant/recessive patterns like Mendel observed.
However, many traits are not inherited following these patterns.
Several other types of inheritance patterns exist:1. Codominance
2. Incomplete Dominance
3. Multiple Alleles
4. Polygenic Inheritance
5. Pleiotropy
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Codominance
Some alleles are codominant.– Both phenotypes are expressed together in a
heterozygote.– This will result in three phenotypes
Examples– Horse color
DR DR is chestnut color DW DW is white color DW DR is palomino-colored (chestnut with white mane and tail)
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Codominance
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Incomplete Dominance
Occurs when the phenotype of the heterozygote is intermediate between the two homozygotes– Appears as if the heterozygotes are blends of the
homozygotes
Snapdragon color– FwFw=white flower– FrFr=red flower– FwFr=pink flower
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Incomplete Dominance
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Sample Problem: Incomplete Dominance
If a pink snapdragon is crossed with a white snapdragon, what phenotypes can result?
What is the probability of each phenotype?
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Solution Pathway: Incomplete Dominance
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Multiple Alleles
Some traits have more than two possible alleles for a single trait Each person can only have two alleles for a given trait because
diploid organisms have only 2 copies of each gene Example: ABO blood types
– 3 alleles for blood type of red blood cells IA = blood type A IB = blood type B i = blood type O, neither type A or type B
– Six possible genotypes; each individual can only have two alleles IAIA, IAi = Type A blood IBIB, IBi = Type B blood IBIA = Type AB blood Ii = Type O blood
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Sample Problem:Multiple Alleles
Allele A and allele B are codominant. Allele A and allele B are both dominant to O. A male heterozygous with blood type A and a
female heterozygous with blood type B have a child.
What are the possible phenotypes of their offspring?
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Solution Pathway:Multiple Alleles
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Polygenic Inheritance
Polygenic = “multiple genes”– some characteristics are determined by the
interaction of several genes Some characteristics are determines by
multiple genes Polygenic inheritance is common with
characteristics that show great variety within the population.– height, eye color, intelligence, etc.
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Skin Color is a Polygenic Trait
Skin color is governed by at least 3 different genes.– Therefore, a wide variety of skin colors exist in the human
population.
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Pleiotropy
Some genes affect multiple phenotypes The disease PKU results from a mutation in
one gene.– The one defective protein leads to several
phenotypes. Mental retardation, abnormal growth, pale skin
pigmentation
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Marfan’s Syndrome is Pleiotropic
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Linkage
Genes that are on the same chromosome are linked
Linked genes are inherited together more often than would be predicted by probability
– because they do not assort independently of each other
All of the genes on a given chromosome represent a linkage group
– all of the genes in a linkage group will be inherited together
Linked genes may be separated during crossing-over of meiosis
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Autosomes
Autosomes - carry genes not involved in sex determination
Of the 23 pairs of human chromosomes, pairs #1-22 are autosomes
Pair #23 are sex chromosomes– X and Y
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Sex Determination in Humans
The sex chromosomes, X and Y, are a homologous pair:– this pair is unique because X and Y carry different
sets of genes– the Y chromosome has genes that determine
maleness– the X chromosome has a variety of genes on it
XX = female; XY = male X_ = female; _Y = does not survive XXY = male
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Sex Determination in Humans
Genotype: X__ Sex: Female Phenotype: Turner’s Syndrome
– short stature (less than 5’)– ‘webbed’ neck and other physical characteristics– infertility
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Sex Determination in Humans
Genotype: XXX Sex: Female Phenotype: ‘Super-females’, metafemales
– tall stature– longer legs and torso– may have learning disabilities or emotionally
underdeveloped– commonly labeled as ‘trouble makers’ in school
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Sex Determination in Humans
Genotype: XYY Sex: Male Phenotype: ‘Super-males’
– produce higher levels of testosterone– may be taller than average– no known significant abnormalities
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Sex Determination in Humans
Genotype: XXY or XXXY Sex: Male Phenotype: Klinefelter Syndrome
– produce very little testosterone– taller and more overweight than average– may have feminine characteristics – sterile or nearly sterile– most have normal cognitive abilities – can be treated with testosterone early in life
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Sex Linkage
Sex-linked gene = genes on the X or Y chromosomes– genes on the X chromosome are called X-linked– genes on the Y chromosome are Y-linked, this is uncommon
Sex-linked genes are expressed differently than autosomal-linked genes
– males only have one X chromosome, so one copy of a recessive allele will result in the recessive phenotype in men
– women have two copies of X, so they can be heterozygous without showing the phenotype
Important X-linked traits include hemophilia, color-blindness, muscular dystrophy
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Sex Linkage
• Important X-linked traits include hemophilia, color-blindness, and muscular dystrophy
- the alleles for these traits are carried on the X chromosome
• Example: Hemophilia h
- Genotype: XXh, phenotype?:
- Genotype: XhXh, phenotype?:
- Genotype: XhY, phenotype?:
• X-linked disease are much more common in males
• Females are carriers of X-linked diseases
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Linked Genes are Found on the Same Chromosome
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Sex Chromosomes
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X-linked Inheritance Patterns
In humans, the allele for normal color vision is dominant and the allele for color deficiency (blindness) is recessive.
Both alleles are X-linked. People who cannot detect the difference between
certain colors such as green and red are described as having color-deficient vision.
A male who has normal color vision mates with a female who is heterozygous for normal color vision.
What type of children can they have in terms of these traits?
What is the probability for each type?
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Solution Pathway:X-linked Inheritance
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Other Influences on Phenotype
Variable expressivity– Some dominant traits
are not expressed equally in all individuals with the trait.
– Polydactylism
Environmental factors– Can influence the
expression of a trait– Freckles and sunlight– Diabetes and diet
Epigenetics