9.6 Geneticists can use the testcross to determine unknown genotypes
§ If we want to know if a black lab is of the BB or Bb variety, we can cross it with a homozygous recessive trait ( bb , the brown lab)
§ If the black lab was BB and we cross it with a bb lab, the outcome should be 100 % Bb labs ….. Or 100 % black labs
§ But if the black lab was of the Bb variety, it can produce gametes with either B or b traits. Mating with a bb individual would thus produce 50% Bb (black) and 50 % bb (brown) labs
9.8 CONNECTION: Genetic traits in humans can be tracked through family pedigrees
Kristin Straight hairline
Lori Widow’s peak
Let’s assign the allele letter for hairline characteristics H or h.
Which of these two traits is dominant and what are the genotypes for this allele in these sisters ? HH, Hh or hh ? Analyzing a pedigree can help us find the answer.
Kristin and Lori are sisters and have a different hairline
9.8 CONNECTION: Genetic traits in humans can be tracked through family pedigrees
Assume the opposite . If both parents have the same characteristic and it is recessive ?
Thus all off spring will show the dominant characteristic. Hence, the only way the offspring shows a recessive trait if bot parents are heterozygous for that trait.
If two parents show the same characteristic, and one of their children shows the opposite characteristic, then the parents characteristic must be dominant and heterozygous !
hh x hh the offspring must all be hh
So the characteristic of the parents must be dominant. What is one is homozygous dominant and the other heterozygous ?
HH x Hh the offspring will be HH or Hh
4
Figure 9.8-2
Mating
Al Beth
Children
Evelyn Frank Gary Henry
Female Male Widow’s peak hairline trait Straight hairline trait
KEY
Charles Debbie
Isabel Juliana
Widow’s peak Straight hairline
Kristin Lori
1ST GENERATION
2ND GENERATION
3RD GENERATION
Parents with same trait but with children having opposite trait
Figure 9.8-3
A parent with a widow’s peak who has a child with a straight hairline must be Hh.
Al Beth
Kristin has a straight hairline but her parents do not, so straight hairline must be homozygous recessive (hh).
Evelyn Frank Gary Henry
Female Male Widow’s peak hairline trait Straight hairline trait
H: widow’s peak allele h: straight allele
KEY
Charles Debbie
Isabel Juliana
Widow’s peak Straight hairline
Kristin Lori
1ST GENERATION
2ND GENERATION
3RD GENERATION
Hh Hh Hh
Hh Hh hh hh
hh
hh
hh
9.8 CONNECTION: Genetic traits in humans can be tracked through family pedigrees
Kristin has a straight hairline but her parents do not, so straight hairline must be homozygous recessive (hh).
Not all genotypes can be determined. Lori could be HH or Hh and there is no way to know (unless she has some children and the pedigree is extended).
5
Human genetics
§ Although dominant traits rule over recessive traits, dominant traits are not always the most common trait in nature
§ Wild-type traits is the term used for traits prevailing (common) in nature ( and are thus not necessarily specified by dominant alleles).
We assumed Not freckled is dominant; that automatically makes having freckles is recessive This diagram shows that the assumption of having freckles being recessive doesn’t work. The genotype of the child cannot be explained using the genotypes of the parents.
No way Mom and Dad can produce a child with NN or Nn genotype if having freckles is recessive.
Thus having freckles is dominant and no freckles is recessive.
9.8 Genetic traits and human family pedigrees
§ Thus freckles is dominant and the child with freckles means, the child is ff
§ What are Mom and Dad in terms of genotype if they have freckles ?
§ Mom and Dad must both be Ff, because that is the only way we can have a child with ff
§ The following are some examples of human traits with dominant and recessive forms.
§ All of these are minor variations with no major side effects, but do obey Mendel’s law of genetics
Human Genetic traits
Tongue rolling is dominant Hitchkiker’s thumb is as well
Bent pinky ( dominant) vs straight pinky
Dimples in cheek ( dominant) vs no dimples
Curly hair ( dominant) vs straight hair
Long eyelashes ( dominant) vs short eyelashes
Left thumb over right thumb when interlacing fingers is dominant
Hair on your middle digits of your fingers is dominant
Human Genetic traits
9.8 Genetic traits and human family pedigrees
§ Due to the fact that recessive traits only show up when the individual is homozygous for the recessive allele, we can figure out who in the family tree is the carrier.
§ For example , when two people, who have a certain trait, have a child that does not have that trait, one may assume that the trait of the child is recessive.
§ Reason ? The only way to have gotten the trait IF both parents have at least one copy of that recessive allele.
Similar reasoning on this female child, tells us that attached earlobes is recessive (her parents had free earlobes). Thus parents must be Ff. Parents cannot be FF and Ff or both be FF !
9.8 Genetic traits and human family pedigrees : Earlobes example
First generation (grandparents)
Second generation (parents, aunts, and uncles)
Third generation (two sisters)
Female Male Attached Free
Ff Ff Ff ff
Ff Ff ff ff ff
ff
FF or Ff
FF or Ff
9.8 CONNECTION: Genetic traits in humans can be tracked through family pedigrees
One of the parents had a grandparent and sibling with attached earlobes. Thus the other grandparent must have been Ff as well.
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9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene
§ Inherited human disorders show either
1. recessive inheritance in which
– two recessive alleles are needed to show disease,
– heterozygous parents are the called carriers of the disease-causing allele, and
– the probability of inheritance increases with inbreeding, mating between close relatives.
2. dominant inheritance in which
– one dominant allele is needed to show disease and
– dominant lethal alleles are usually eliminated from the population.
This shows an example of a recessive form of deafness. Normal hearing is thus the dominant allele, deafness due to a mutation in the gene. Only people with both mutated genes become deaf (dd).
9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene
§ The most common fatal genetic disease in the United States is cystic fibrosis (CF), resulting in excessive thick mucus secretions. The CF allele is
– recessive and an indiviudal must be homozygous recessive (aa) to get the disorder
– carried by about 1 in 31 Americans. ( they are heterozygous (Aa )
§ Dominant human disorders include (so, if an individual has the dominant allele , they will get the disorder ( AA or Aa)
– achondroplasia, resulting in dwarfism, and
– Huntington’s disease, a degenerative disorder of the nervous system.
Dr. Michael C. Ain, a specialist in the repair of bone defects caused by achondroplasia and related disorders. He himself has achondroplasia. So his genotype is either AA or Aa for this trait. The only way he could have gotten this if either both his parents had the disorder or one parent did not and the other did Parents : AA x AA Kids : all AA Parents : AA x Aa Kids : 2 AA, 2 Aa Parents : AA x aa Kids : all Aa Parents : Aa x Aa Kids : 1 AA, 2 Aa, 1 aa Parents : Aa x aa Kids : 2 Aa, 2 aa Parents : aa x aa Kids : all aa Those without the disorder in Blue
9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene
Table 9.9
§ New technologies offer ways to obtain genetic information – before conception,
– during pregnancy, and
– after birth.
§ Genetic testing can identify potential parents who are heterozygous carriers for certain diseases.
9.10 CONNECTION: New technologies can provide insight into one’s genetic legacy
§ Blood tests on the mother at 14–20 weeks of pregnancy can help identify fetuses at risk for certain birth defects.
§ Fetal imaging enables a physician to examine a fetus directly for anatomical deformities. The most common procedure is ultrasound imaging, using sound waves to produce a picture of the fetus.
§ Newborn screening can detect diseases that can be prevented by special care and precautions.
9.10 CONNECTION: New technologies can provide insight into one’s genetic legacy
§ Mendel’s pea crosses always looked like one of the two parental varieties, a situation called complete dominance.
§ For some characters, the appearance of F1 hybrids falls between the phenotypes of the two parental varieties. This is called incomplete dominance.
9.11 Incomplete dominance results in intermediate phenotypes
• One example of incomplete dominance in humans is hypercholesterolemia, in which genes code for cholesterol (LDL) receptors
• dangerously high levels of cholesterol occur in the blood and • heterozygotes have intermediately high cholesterol levels.
Phenotypes
Mild disease Severe disease Normal Cell
LDL
LDL receptor
Genotypes HH Homozygous
for ability to make LDL receptors
Hh Heterozygous
hh Homozygous
for inability to make LDL receptors
9.11 Incomplete dominance results in intermediate phenotypes
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9.12 Many genes have more than two alleles in the population
§ Although each individual carries, at most, two different alleles for a particular gene, in cases of multiple alleles, more than two possible alleles exist in a population.
§ For example, a certain trait has 3 flavors, A, B and C floating around in a population
§ So, possible combinations for alleles in an individual could be
– AB or AC or BC ( remember that only two alleles can occur at same time)
– Think like socks on your feet ( you have many pair but only two socks fit on your feet at any time…. Many combinations possible)
9.12 Many genes have more than two alleles in the population
§ Human ABO blood group phenotypes involve three alleles for a single gene.
§ The four human blood groups, A, B, AB, and O, result from combinations of these three alleles.
§ The A and B alleles are both expressed in heterozygous individuals, making both alleles codominant.
§ The alleles code for carbohydrate groups on red blood cells
9.12 Many genes have more than two alleles in the population
§ The alleles are coded as follow
– Allele IA codes for carbohydrates A on RBC
– Allele IB codes for carbohydrates B on RBC
– Allele IO codes for no A or B carbohydrates on RBC ( sometimes referred to as allele i )
§ So what are the possible combinations ?
14
Carbohydrate A
Carbohydrate B
Carbohydrate A and Carbohydrate B
Neither
Blood Group (Phenotype) Genotypes
Carbohydrates Present on Red Blood Cells
A
O
B
AB
IAIA or IAi
IBIB or IBi
IAIB
ii
§ Modern Paternal testing uses DNA analysis, but this knowledge of blood groups makes for some quick and easy determinations.
§ For example, a woman claims a man is the father of her baby. She is AB blood type and the baby is blood type A. He claims he is blood type O and thus cannot be the Dad. True or False ?
9.12 Many genes have more than two alleles in the population
§ MOM = AB = IAIB
§ DAD = O = ii
§ So possible genotypes (phenotypes) of children
IA IB
i IAi IBi
i IAi IBi
Children will have either A or B blood type.
9.12 Many genes have more than two alleles in the population
15
9.13 A single gene may affect many phenotypic characters
§ Pleiotropy occurs when one gene influences multiple characters.
§ Sickle-cell disease is a human example of pleiotropy.
– This disease affects the type of hemoglobin produced and the shape of red blood cells and causes anemia and organ damage.
– Sickle-cell and nonsickle alleles are codominant.
– Carriers of sickle-cell disease have increased resistance to malaria.
An individual homozygous for the sickle-cell allele
Produces sickle-cell (abnormal) hemoglobin
The abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped
Pain and fever Joint problems Physical weakness Anemia Pneumonia and other infections
9.14 A single character may be influenced by many genes
• Many characters result from polygenic inheritance, in which a single phenotypic character results from the additive effects of two or more genes on a single phenotypic character.
• Human skin color is an example of polygenic inheritance.
9.14 A single character may be influenced by many genes
9.15 The environment affects many characters
§ Many characters result from a combination of heredity and the environment. For example,
– skin color is affected by exposure to sunlight and
– heart disease and cancer are influenced by genes and the environment.
§ Identical twins show that a person’s traits are the results of
– genetics and
– the environment.
9.20 Chromosomes determine sex in many species
§ Many animals have a pair of sex chromosomes, designated X and Y, that determine an individual’s sex.
§ Among humans and other mammals,
– individuals with one X chromosome and one Y chromosome are males, and
– XX individuals are females.
§ In addition, human males and females both have 44 autosomes (nonsex chromosomes).
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9.20 Chromosomes determine sex in many species
§ In mammals (including humans),
– the Y chromosome has a crucial gene, SRY, for the development of testes, and
– an absence of the SRY gene directs ovaries to develop.
X
Y
9.20 Chromosomes determine sex in many species
§ In certain fishes, butterflies, and birds, the sex chromosomes are Z and W,
– males are ZZ, and females are ZW.
§ Some organisms lack sex chromosomes altogether.
§ In most ants and bees, sex is determined by chromosome number.
– Females develop from fertilized eggs and thus are diploid.
– Males develop from unfertilized eggs. Males are thus
– fatherless and haploid.
Chromosome number
determines sex
9.20 Chromosomes determine sex in many species
§ In some animals, environmental temperature determines the sex.
– For some species of reptiles, the temperature at which the eggs are incubated during a specific period of embryonic development determines whether the embryo will develop into a male or female.
– Global climate change may therefore impact the sex ratio of such species.
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9.21 Sex-linked genes exhibit a unique pattern of inheritance
§ Sex-linked genes are located on either of the sex chromosomes.
§ The X chromosome carries many genes unrelated to sex.
§ Most sex-linked human disorders are
– due to recessive alleles and
– seen mostly in males
9.22 CONNECTION: Human sex-linked disorders affect mostly males
§ A male receiving a single X-linked recessive allele from his mother will have the disorder.
§ A female must receive the allele from both parents to be affected.
§ Recessive and sex-linked human disorders include
– hemophilia, characterized by excessive bleeding because hemophiliacs lack one or more of the proteins required for blood clotting,
– red-green colorblindness, a malfunction of light-sensitive cells in the eyes, and
– Duchenne muscular dystrophy, a condition characterized by a progressive weakening of the muscles and loss of coordination.
Albert Queen Victoria
Alice Louis
Alexandra Czar Nicholas II of Russia
Alexis
Female Male Hemophilia
Carrier
Normal
9.22 CONNECTION: Human sex-linked disorders affect mostly males
Pedigree of Hemophelia in Royal Russian family
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9.23 EVOLUTION CONNECTION: The Y chromosome provides clues about human male evolution
§ The Y chromosome provides clues about human male evolution because
– Y chromosomes are passed intact from father to son and
– mutations in Y chromosomes can reveal data about recent shared ancestry.
§ In 2003, geneticists discovered that about 8% of males currently living in central Asia have Y chromosomes of striking genetic similarity.
9.23 EVOLUTION CONNECTION: The Y chromosome provides clues about human male evolution
§ Further analysis traced their common genetic heritage to a single man living about 1,000 years ago.
§ In combination with historical records, the data led to the speculation that the Mongolian ruler Genghis Kahn may be responsible for the spread of the telltale chromosome to nearly 16 million men living today
