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Complex Patterns of Heredity
Chapter 12
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Human Inheritance
• Humans have 23 pairs of chromosomes
• These are made of about 100,000 genes
• Scientists usually study disease causing genes because they can easily be traced
• They often prepare a Pedigree – a family record that shows how a trait is inherited over several generations.
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Pedigree
• Carriers – usually, Heterozygous; they do not express the recessive allele, but they pass it along to their offspring.
Square = Male
Circle = Female
No shading = normal
Shaded = displays trait
Half/Half = Carrier
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PedigreeA Pedigree of Hemophilia in the Royal Families of Europe
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Simple Dominant Heredity
• Many traits are inherited just as the rule of dominance predicts.
• Remember that in Mendelian inheritance, a single dominant allele inherited from one parent is all that is needed for a person to show the dominant trait.
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Patterns of Inheritance
• Phenotypes repeated in a predictable pattern from generation to generation
• Genetic Disorders – diseases or debilitating conditions that have a genetic basis
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Patterns of Inheritance
• Traits controlled by a Single Allele– More than 200 human traits are determined by
a single dominant allele• Ex. Huntington’s Disease
– More than 250 other traits are determined by homozygous recessive alleles
• Both parents must have this allele in order for their offspring to have the disease.
• Ex. Cystic Fibrosis & Sickle cell anemia
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Inheritance Patterns• Recessive Allele Inheritance Patterns
– Unaffected parents can have affected offspring– The phenotype can skip a generation– Individuals with no signs of the allele can be
carriers
• Dominant Allele Inheritance Patterns– Affected offspring must have at least one affected
parent– The phenotype appears in every generation without
skipping– Two unaffected parents have no affected offspring
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Sometimes Heredity Follows Different Rules
• Incomplete Dominance: Appearance of a third phenotype
• Codominance: Expression of both alleles
• Multiple phenotypes from multiple alleles
• Sex determination
• Sex-linked inheritance
• Polygenic Inheritance
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Incomplete Dominance• With incomplete dominance, a
cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits. – RED Flower x WHITE Flower ---
> PINK Flower
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• R = allele for red flowers W = allele for white flowers red x white ---> pink RR x WW ---> 100% RW
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Codominance• The genetic gist to codominance is pretty
much the same as incomplete dominance. A hybrid organism shows a third phenotype --- not the usual "dominant" one & not the "recessive" one ... but a third, different phenotype.
• In COdominance, the "recessive" & "dominant" traits appear together in the phenotype of hybrid organisms.
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Codominance
• Example
red x white ---> red & white spotted
With codominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together.
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• R = allele for red flowers W = allele for white flowers red x white ---> red & white spotted RR x WW ---> 100% RW
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Examples of Codominance• A very very very very very common
phenotype used in questions about codominance is roan fur in cattle. Cattle can be red (RR = all red hairs), white (WW = all white hairs), or roan (RW = red & white hairs together). A good example of codominance. Another example of codominance is human blood type AB, in which two types of protein ("A" & "B") appear together on the surface of blood cells.
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Codominance and Blood Types• Blood transfusion can only take place
between two people who have compatible types of blood.
• Human blood is separated into different classifications because of the varying proteins on the surface of blood cells.
• These proteins are there to identify whether or not the blood in the individual's body is it's own and not something the immunity system should destroy.
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Patterns of Inheritance
• Traits controlled by Multiple Alleles– Controlled by 3 or more alleles of the same
gene that code for a single trait– Ex. Blood types
• A & B are codominant – both are expressed when together; and both are dominant to O.
• A person can only have type O blood if they receive the “O” allele from both parents.
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Human Blood Types
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ABO Blood type and genetics
• The protein's structure is controlled by three alleles;
• i, IA and IB. – i, the recessive of the three, – IA and IB are both codominant when paired together. – If the recessive allele i is paired with IB or IA, it's expression is hidden and is not shown. – When the IB and IA are together in a pair, both proteins A and B are present and
expressed.
• The individual's blood type is determined by which combination of alleles he/she has. – There are four possible blood types in order from most common to most rare: O, A, B
and AB. – O blood type represents an individual who is homozygous recessive (ii) and does not
have an allele for A or B.
• Blood types A and B are codominant alleles. – Codominant alleles are expressed even if only one is present. The recessive allele i for
blood type O is only expressed when two recessive alleles are present. – Blood type O is not apparent if the individual has an allele for A or B.– Individuals who have blood type A have a genotype of IAIA or IAi and those with blood
type B, IBIB or IBi,– An individual who is IAIB has blood type AB.
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Blood type practiceUse a Punnett Square!
1. A woman has type A blood. Her father has type O blood. The woman marries a man with type O blood. What is the chance that they will have a child with type A blood?
2. What is the chance that the couple from question 1 will have a child with type AB blood?
*Show me your answers when you are finished. Keep these in your notes!
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Multiple Phenotypes from Multiple Alleles
• Although each trait that we have studied so far only has two alleles, it is common for more that two alleles to control a trait in a population
• For instance, Pigeons – three colors possible (red, blue, chocolate)
• However, each pigeon can have only two of these alleles
• Complete P.S. Lab 12.2 to observe multiple alleles in how coat color in rabbits is inherited.
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Sex determination
• Remember that in humans the diploid number of chromosomes is 46, or 23 pairs.
• There are 22 matching pairs of homologous chromosomes called autosomes.
• The 23rd pair differs in males and females, they determine the sex of an individual (sex chromosomes) – X females (XX)– Y males (XY)
Complete a punnett square to determine the expected ratio of males to females produced given their possible gamete contribution
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Sex-linked inheritance
• Traits controlled by genes located on sex chromosomes are called sex-linked traits
• Read about Thomas Hunt Morgan’s research with fruit flies on pg 325. Complete a punnett square to show how the allele for red eye color is a sex-linked trait.
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Patterns of Inheritance
• Polygenic Traits– Most human characteristics are controlled by
several genes (2 or more)– Ex. Skin color – 3 to 6 genes– Ex. Eye color– Some are also affected by the environment
• Ex. Height – nutrition and disease
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Eye Color Activity
• http://www.athro.com/evo/gen/genefr2.html
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Patterns of Inheritance
• Sex-Linked Traits– Are found only on the X chromosome– Ex. Colorblindness (recessive)– Ex. Hemophilia (recessive)
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Patterns of Inheritance
• Sex-Influenced Traits
• Influenced by male or female sex hormones
• Ex. Patterned Baldness– Homozygous baldness-both will lose hair– Heterozygous-men will lose hair but women
will not
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Nondisjunction
• Nondisjunction is the failure of homologous chromosome pairs to separate properly during meiosis. The result of this error is a cell with an abnormal (too few or too many) number of chromosomes.
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Nondisjunction
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Patterns of Inheritance
• Disorders due to Nondisjunction– Monosomy (45 Chromosomes)– Trisomy (47 Chromosomes)
• Trisomy-21 (Down’s Syndrome)
– Klinefelter’s (XXY)-male w/ some female traits– Turner’s (XO)-female appearance– Single Y chromosome do not survive
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Environmental Effects
• Genes are inherited from parents, but sometimes their expression is modified by environmental factors.
• An example is the snowshoe hare we discussed earlier in the year-these hares have dark fur in the summer and white fur in the winter.
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Snowshoe Hare
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Detecting Human Genetic Disorders
• Genetic Screening – examination of genetic makeup– Karyotype: a picture of chromosomes
grouped in pairs and arranged in sequence.
– Screening of Blood: look for certain proteins• Genetic Counseling-medical guidance
informing of problems that could affect their offspring.
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Human Karyotype
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Detecting HumanGenetic Disorders
– Amniocentesis: removal of small amount of amnionic fluid surrounding the fetus
– Chorionic Villi Sampling: tissue that grows between the mother’s uterus and the placenta (between the 8th and 10th week)
– Screening Immediately after Birth:• Ex PKU (Phenylketonuria)-body cannot
metabolize the amino acid phenylalanine– Special diet lacking phenylalanine
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Karyotyping
• http://www.biology.arizona.edu/human_bio/activities/karyotyping/karyotyping2.html