1. PATTERNS OF INHERITANCE (MENDEL AND GENETICS)
2. GREGOR MENDEL
a. Originally Johann Mendel (took the name Gregor when he entered the Augustinian
brotherhood),
b. Mendel grew up on his parent’s small farm in a region of Austria.
c. He received agricultural training in his home town school along with a basic education.
3. GREGOR MENDEL
a. In 1857, Mendel began breeding garden peas in the abbey of the monastery that he
lived to study inheritance.
b. Mendel had an experimental and quantitative approach to his study of inheritance – he
had new ideas about old questions.
4. IMPORTANT GENETICS DEFINITIONS
a. CHARACTER: A term used by geneticists for a heritable feature, such as flower color,
that varies among individuals. Each variation of a character is a trait, such as purple or
white flower color.
5. GENETICS TERMS
a. Homologous chromosomes: two chromosomes that is similar in appearance and
genetic information. One is received from each parent in a diploid organism.
b. A gene is a unit of heredity that encodes a particular trait.
c. Genes for each trait can be found at a particular locus or physical location along
chromosomes. (The gene’s address.)
6. GENETICS TERMS
a. Alleles are different “flavors” of genes. For example, human blood types are produced
by three different alleles (A, B, O) of the blood type gene. Which two alleles you have
determines if you are blood type A, B, AB, or O.
7. GENETICS TERMS
a. If both homologous chromosomes in an organism have the same allele at a given gene
locus, they are said to be homozygous (“same pair”).
b. If two homologous chromosomes have different alleles at a given gene locus they are
heterozygous (“different pair”). This is sometimes called a hybrid.
8. Gamete
a. A haploid sex cell, such as a sperm or an egg will receive only one of the homologous
chromosomes, and therefore, only one of the alleles.
9. SUCCESSFUL EXPERIMENTATION
a. Three steps to Mendel’s success:
i. Choose correct organism to work with.
ii. Perform experiment correctly
iii. Analyze data properly.
b. Step one – Mendel chose a self-fertilizing plant. This flower can be pollinated by other
flowers via wind or insects.
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10. THE CORRECT ORGANISM TO STUDY
a. PEA FLOWERS SELF-FERTILIZE: the union of sperm and egg from the same
individual.
b. True-breeding: pertaining to an individual all of whose offspring produced through selffertilization are identical to the parental type. True-breeding individuals are homozygous
for a given trait.
11. MENDEL’S SUCCESS
a. Cross-fertilization: the union of sperm and egg from TWO individuals of the same
species.
12. WHAT HAPPENED TO THE WHITE FLOWER?
a. Any generation following the parental generation is a “filial”. The first generation is
called the first filial, or the F1 generation.
b. In the F1 generation, the white flower seemed to disappear.
13. MENDEL CONTINUED…
a. Mendel collected the seeds from the F1 generation and planted them the next spring.
b. The next generation, F2 produced approximately 3/4 purple flowers and ¼ white
flowers, or a 3:1 ratio.
c. The gene for the white flower had been “hidden” for a generation. Where?
14. INHERITANCE OF DOMINANT AND RECESSIVE ALLELES
a. Each individual has two alleles for a given gene.
b. One allele of the gene is present on each homologous chromosome.
c. True breeding peas with white flowers have different alleles of the “flower-color” gene
than true breeding purple-flowered peas.
15. INHERITANCE OF DOMINANT AND RECESSIVE ALLELES
a. The pairs of genes on homologous chromosomes separate from each other when
gametes are formed.
b. Each gamete receives only one allele of an organism’s pair of genes.
c. MENDELS LAW OF SEGREGATION: Each gamete receives only one of each parent’s
pair of genes for each trait.
16. INHERITANCE OF DOMINANT AND RECESSIVE ALLELES
a. WHICH ALLELE BECOMES INCLUDED IN A GAMETE IS DETERMINED BY
CHANCE.
b. SEPARATION OF HOMOLGOUS CHROMOSOMES DURING MEIOSIS IS RANDOM.
c. Probability: the chance that each outcome of an event will occur is proportional to the
number of ways in which that outcome can occur.
17. INHERITANCE OF DOMINANT AND RECESSIVE ALLELES
a. When two different alleles are present in an organism, one may mask the expression of
the other.
b. Dominant: an allele that can determine the phenotype of heterozygotes completely,
such that they are indistinguishable from individuals homozygous for the allele. In the
heterozygote, the expression of the other allele (recessive) is completely masked.
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18. INHERITANCE OF DOMINANT AND RECESSIVE ALLELES
a. Recessive: an allele that is expressed only in homozygotes and is completely masked in
heterozygotes.
b. Note: the dominant allele does not alter the physical presence of the recessive allele.
c. TRUE-BREEDING organisms have two of the same alleles for a given gene and are
called “homozygous”.
19. INHERITANCE OF DOMINANT AND RECESSIVE ALLELES
a. By Mendel’s convention, the capital letter represents the DOMINANT ALLELE.
b. Here seen as P.
c. The lowercase letter p is the recessive allele, in this case representing white flower
color.
d. True-breeding or homozygous plants will have two of the same letters (representing
gametes).
e. PP for purple, pp for white.
20. INHERITANCE OF DOMINANT AND RECESSIVE ALLELES
a. GENOTYPE: the genetic composition of an organism; the actual alleles of each gene
carried by the organism.
b. Example: PP for true-breeding purple.
c. PHENOTYPE: the physical characteristics of an organism; can be defined as outward
appearance (such as flower color), as behavior, or in molecular terms (such as antigens
on red blood cells).
21. PUNNETT SQUARE METHOD:
a. An intuitive way to predict the genotypes and phenotypes of offspring in specific
crosses. “Genetic bookkeeping”.
b. Named after R.C. Punnett, a famous geneticist of the early 1900’s.
c. Let’s look at Mendel’s experiments through a Punnett square.
22. LOOK AT FLOWERS TO REMEMBER WHAT WE ARE TALKING ABOUT.
a. Monohybrid: Individuals heterozygous for one gene; P/p
b. Monohybrid experiment: Cross between two identical monohybrids:
c. P/p x P/p
23. Plants with either PP or Pp genotypes make purple flowers.
a. These plants have different GENOTYPES, but they have the SAME PHENOTYPE.
b. Genotypic ratio in the F1 generation is 1:2:1(including white, pp)
c. Phenotypic ratio in the F1 generation is 3:1
24. GENOTYPE AND PHENOTYPE RATIOS
a. When Mendel let the F1 generation self-fertilize (Pp x Pp), this was the result.
b. This cross represents the same type of cross that occurs in human’s who are carriers of
disease that is autosomal recessive.
25. CYSTIC FIBROSIS
a. “Woe be to the child which when kissed on the forehead tastes salty. He is bewitched
and soon must die.” -17th century English saying.
b. Cystic fibrosis is an autosomal recessive disorder caused by >900 known mutations on
the CFTR gene on chromosome 7.
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c. Predominantly a disease of northern Europeans with an approximate carrier rate of 1/25
in Caucasians.
d. Gentle pounding on chest relieves mucus. Infusion of antibiotic through the wrist.
26. CYSTIC FIBROSIS
a. Reduced chloride secretions create thick mucus
b. The cilia of the lungs can not move the secretions well, and it clogs the airways.
c. This also means that bacteria remain in the lungs, causing frequent infections.
27. HOW DO WE TEST UNBORN FETUSES?
28. TEST CROSS
a. Test cross: A breeding experiment in which an individual showing the dominant
phenotype is mated with an individual that is homozygous recessive for the same gene.
b. The ratio of offspring with dominant versus recessive phenotypes can be used to
determine the genotype of the phenotypically dominant individual.
29. OFFSPRING OCCUR IN APPROXIMATE PROPORTIONS
a. In a real experiment, the offspring will occur only in approximately the predicted
proportions.
i. Some sperm do not fertilize an egg, and some eggs are not fertilized by any
sperm.
ii. Consider male and female offspring. Do parents always have 50% male, 50%
female?
30. MULTIPLE TRAITS
a. Mendel then began to study multiple traits in peas.
b. He began by cross-breeding plants that differed in two traits, for example:
i. Seed color (yellow or green)
ii. Seed shape (smooth or wrinkled)
31. MULTIPLE TRAITS
a. Mendel knew from previous crosses that the smooth (S) allele of the seed shape gene
was dominant to the wrinkled (s) seed shape allele.
b. The yellow allele of the seed color gene (Y) is dominant to the green allele (y).
c. He began by crossing true breeding plant with smooth yellow seeds with a truebreeding plant with wrinkled, green seeds.
d. What did he end up with in the first generation?
32. Mendel then permitted the F1 generation to self-fertilize. This represented a cross between
organisms HETEROZYGOUS for two traits. Two independent 3:1 phenotypic ratios combine to
produce an overall 9:3:3:1 ratio.
a. LAW OF INDEPENDENT ASSORTMENT: the independent inheritance of two or more
distinct traits; states that the alleles for one trait may be distributed to the gametes
independently of the alleles for other traits.
33. EXAMPLE OF INDEPENDENT ASSORTMENT - see cross of heterozygote SsYy by
chromosome.
34. GENETIC LINKAGE
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a. Genetic linkage is the inheritance of certain genes as a group because they are on the
same chromosome, close together.
b. Linked genes do not show independent assortment.
c. Crossing over: the exchange of corresponding segments of the chromatids of two
homologous chromosomes during meiosis.
35. SEX DETERMINATION
a. In mammals and many insects, males have the same number of chromosomes as
females do, but one “pair”, the sex chromosomes, is very different in appearance.
b. Females have two X chromosomes.
c. Males have one X and one Y chromosomes.
d. The Y chromosomes carriers relatively few genes. Approximately 78 have been found.
e. The X and the Y chromosome act as homologous chromosomes during meiosis.
36. MALES DETERMINE SEX
a. All chromosomes that are not sex chromosomes are called autosomes and males and
females have the same pairs.
b. How many autosomes are there?
c. If a male is affected with an X-linked disorder, how many of are his sons are affected?
37. PATTERN OF SEX DETERMINATION IN HUMANS
a. An early human embryo appears neither male nor female.
b. Then, ducts and other structures that develop into male or female reproductive organs
form.
c. In an XX embryo, ovaries form in the absence of the SRY gene on the Y chromosome.
d. In an XY embryo, the gene product causes testes to form. Testes prompt the
development of other male traits.
38. X-LINKED GENES
a. Called sex-linked genes in textbook.
b. Express themselves as dominant in males, heterozygous in females. Why?
c. Can males be heterozygotes for X-linked diseases? WHAT ARE THEY??
d. Because females have two X chromosomes, they can be homozygous or heterozygous
for genes on the X chromosome, and dominant versus recessive alleles will be
expressed.
39. X-LINKED EYE COLOR
a. Drosophila melanogaster commonly used. One mating = hundreds of offspring every
two weeks.
b. Normally, Drosophila has red eyes.
c. In the early 1900’s, researchers in the lab of Thomas Hunt Morgan of Columbia
University found a white eyed male fruit-fly with unusual Punnet square results.
40. COLOR-BLINDNESS IN HUMANS
a. What do you see in this picture?
b. THIS IS AN ISHIHARA CHART, NAMED AFTER ITS INVENTOR.
i. If colors on screen are accurate, and you have normal vision, you will see a 96.
ii. If you have red-deficient vision, you will see a 6.
iii. If you have green-deficient vision, you will see a 9.
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41. HEMOPHILIA – ROYAL HOUSE OF EUROPE
a. Queen Victoria (with cane) in 1885 and her offspring.
b. Queen Victoria‘s ancestors were free of hemophilia, and therefore, the mutation she
passed on to virtually every royal house in Europe either arose as a new mutation in
her, in her mother (father unaffected).
c. 1/3 of hemophilia patients with no family history have a new mutation.
42. HEMOPHILIA IN THE ROYAL FAMILY IS X-LINKED
a. Hemophilia is a “bleeding” disease.
b. Mutations in the F8C or F9 gene cause a deficiency or dysfunction of clotting factors VIII
or IX. No clotting factor – you keep bleeding.
c. About 1/5000-1/10,000 newborn males are affected with Hemophilia A.
d. Females can be affected – How?
43. HEMOPHILIA FAMILY TREE ROYAL FAMILY.
44. OTHER X-LINKED DISORDERS:
1. Pedigree nomenclature has been standardized in the medical community
since 1995.
2. Other X-linked disorders you may have heard about:
3. Muscular dystrophy
4. Fragile X mental retardation
45. INCOMPLETE DOMINANCE
a. Every trait that is inherited is not necessarily controlled by a single gene.
i. Many traits are influenced by several genes.
ii. Incomplete dominance is an example.
b. Incomplete Dominance: a pattern of inheritance in which the heterozygous phenotype is
intermediate between the two homozygous phenotypes.
46. INCOMPLETE DOMINANCE
a. In genetic nomenclature, incompletely dominant alleles are often given uppercase
symbols with a superscript to denote different alleles.
b. Phenotypic ratio in incomplete dominance is the same as genotypic ratio. Why?
47. MULTIPLE ALLELES AND CODOMINANCE
a. A diploid organism can have only 2 alleles as part of their genotype at a certain locus
even though species can have multiple alleles of many of its genes.
b. Drosophila has more than 1000 alleles for eye color.
48. MULTIPLE ALLELES AND CODOMINANCE
a. Multiple Alleles: as many as dozens of alleles produced for every gene as a result of
different mutations.
b. Codominance: the relation between two alleles of a gene, such that both alleles are
phenotypically expressed in heterozygous individuals.
49. Human blood groups are a good example of multiple alleles and codominance.
a. A person’s blood cells are coated with one substance or the other, or both, or neither.
Those substances, called glycoproteins in your book, create your blood type.
b. There are 4 phenotypes of blood type. A, B, AB, O
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c. These are created by 3 alleles: There are 3 different alleles.
d. IA, IB, i. IA stands for the A glycoprotein, IB stands for the B glycoprotein, i means there
was neither.
e. There are 6 genotypes. How does it work?
50. POLYGENIC INHERITANCE
a. Many traits are influenced by SEVERAL genes.
b. Human height, weight, eye color, skin color.
c. Continuous variation – can not be split up into conventional, easily defined inheritance
patterns.
d. At least three or four genes control skin pigmentation in people. Exposure to the sun
further alters skin color. Humans demonstrate virtually endless variation for this trait.
51. POLYGENIC INHERITANCE
a. Grain color of wheat is a good example of polygenic inheritance.
b. At least two separate genes, each with two incompletely dominant alleles, determine
wheat grain color.
c. A cross between two wheat plants, both heterozygous for both genes, produces 5
colors of offspring.
52. Pleiotropy: A situation in which a single gene influences more than one phenotypic
characteristic.
a. Example: SRY gene, discovered on the Y chromosome in 1990.
b. SRY – “sex-determining region”.
c. This gene codes for a protein that activates other genes. Those genes in turn for
proteins that witch on male development in an embryo.
d. Due to mutations, some males can be phenotypically females because they are missing
the SRY gene from their Y chromosome.
53. NATURE vs. NURTURE
a. Heredity and environment play major roles in the expression of traits.
b. Examples: Himalayan rabbit AND Siamese cat – has pale body fur but black ears, nose,
tail, feet.
c. The Himalayan rabbit actually has the genotype for black fur all over its body. However,
the enzyme that produces the black pigment is inactive at temperatures above 34
degrees Celsius (93 degrees Fahrenheit).
d. At typical ambient temperatures, extremities such as the ears and feet are cooler than
the rest of the body, so black pigment can be produced there.
e. Because the rabbit’s main body surface is warmer than 34 degrees Celsius, the fur
there is pale.
54. Height
a. Also demonstrates multifactorial inheritance.
b. Is greatly affected by nutrition.
c. Also, external influences such as smoking.
d. Intelligence – Twins separated at birth have similar scores on their IQ tests – but not as
similar as Twins raised in the same home.
55. HOW DO WE INVESTIGATE HUMAN GENETIC DISEASE?
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a.
b.
c.
d.
Experimental crosses with humans are poorly received by most of the world.
Geneticists search medical, historical and family records to study PAST crosses.
Family pedigrees are a useful tool to follow family patterns.
Pedigrees help to predict risk, and now guide medical professionals in testing
individuals at risk for a disorder.
56. PEDIGREES
a. Important symbols to read pedigrees:
i. Male, Female, Sex unknown, Death
ii. Generation Notation and Individual in Generation
iii. Relationship Line (don’t forget consanguinity!), multiple marriages.
iv. Line of descent, Adoption, SAB, EAB
v. Sibship Line, Twins No children, Infertility
vi. Individual’s Lines
vii. Sperm or Ovum Donor, Surrogates, Intended Parents.
57. AUTOSOMAL RECESSIVE DISORDERS
a. ALREADY REVIEWED – CYSTIC FIBROSIS
b. ALBINISM: Controlled by a single recessive gene called TYR. The enzyme tyrosinase is
needed to produce melanin, the dark pigment in skin cells. If individual is homozygous
for the mutant allele, they will be an albino.
58. SICKLE CELL ANEMIA IS CAUSED BY A SINGLE SUBSTITUTION OF ONE NUCLEOTIDE
IN ONE AMINO ACID.
a. 8% of the African-American population is heterozygous for this mutation. Reflects a
genetic legacy of African Origin.
b. In some regions of Africa, 15-20% of population carries the allele. Mediterranean,
Middle Eastern, Central or South American, and East Indian ancestry also increases
risk.
c. Malaria resistance is likely the reason for mutations high prevalence where disease risk
is high.
59. AUTOSOMAL DOMINANT
a. Most harmful alleles are recessive.
b. Fewer human disorders are due to dominant alleles.
c. Achondroplasia, a form of dwarfism (1/10,000 people) is dominantly inherited.
Therefore, 9,999/10,000 people are homozygous recessive for this allele.
d. Can affected individuals have children without dwarfism?
e. High rate of new mutations in autosomal dominant disorders.
60. NANCY WEXLER and HUNTINGTON’S DISEASE
61. TOTAL CHROMOSOME NUMBER NEEDS TO BE CORRECT
a. Genetic disorders can be caused by the wrong number of chromosomes as we have
seen in Down syndrome.
b. These errors occur in meiosis and are called nondisjunction.
c. Nondisjunction: an error in meiosis in which chromosomes fail to segregate properly into
the daughter cells.
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62. GENETIC DISORDERS CAUSED BY ABNORMAL NUMBERS OF SEX CHROMOSOMES
63. STRUCTURAL CHANGES IN CHROMSOMES
a. Duplications
b. Deletions
c. Inversions
d. Translocations
64. CRI-DU-CHAT
a. These individuals have a distinctive cat like mewing for a cry by infants.
b. Microcephaly (abnormally small head), moonlike face, mental retardation.
c. Accounts for 1% of all institutionalized mentally retarded patients.
d. Most cases are sporadic; 10-15% of the patients are offspring of translocation carriers.
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