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Lecture Learning Tools Hub |
Students are required to read the textbook for the instructor to be able to teach the concepts the course encompasses. (From FOUNDATION SKILLS in the Biology Department Master Syllabus.)
Students, do not just read these notes, to succeed in this class -- at least if you wish to have a high grade, you MUST use your textbook as a LEARNING TOOL!
MAIN TOPICS OUTLINE
10.1 INTRODUCTION
10.11 Father of genetics
10.12 Chromosomes
10.13 Determination of Characteristics
10.2 SIMPLE GENETIC TRAITS (main topic 1)
10.21 Terminology
10.22 Natural characteristics (normal traits)
10.23 Genetic disorders
10.3 COMPLEX GENETIC TRAITS (main topic 2)
10.31 Complex gene patterns
10.32 Complex chromosomal patterns
10.4 ENVIRONMENTAL INFLUENCES
LECTURE OBJECTIVES:
1. Differentiate between genotype and phenotype.
2. Work single-factor (monohybrid) genetic problems dealing with traits that show dominance and recessiveness.
3. Explain the Principle of Segregation.
4. Explain the Principle of Independent Assortment.
5. Be able to design a Punnett square to predict reproductive outcomes from different parents.
6. Explain how environmental conditions influence an organism’s phenotype.
7. Define selected key terms.
Key Terms
def. Allele: Alternative forms of a gene for a particular characteristic.
def. Autosomes: Chromosomes not determining the sex of an individual.
def. Chromosome: long molecule of DNA & proteins, that contains the genes, found in the nucleus of eukaryotic cells
def. Dihybrid cross: A mating cross between two individuals that differ in TWO traits.
def. Diploid cell: A cell that has two sets of chromosomes -- one from the maternal parent, and one set from the paternal parent.
def. DNA: Molecule of inheritance; Natural organic molecule, structure double stranded, and helical, capable of replicating and determining the inherited structure of a cell's proteins.
def. Dominant allele: An allele that always expresses itself and masks the effect of a recessive homologous allele.
def. Down Syndrome: A genetic disorder resulting from the presence of an extra chromosome number 21.
def. Fertilization: Joining of two haploid nuclei, a sperm and an egg, resulting in a diploid cell, the zygote.
def. Gene: Unit of heredity, the portion of DNA that determines characteristics (traits) by coding for a particular protein.
def. Genetics: The scientific study of heredity and variation; the study of genes, how genes produce characteristics (traits that vary), and how the characteristics are inherited.
def. Genotype: The genetic constitution of an organism -- distinguished from its physical appearance.
def. Haploid cell: A cell that has a single set of chromosomes -- in eukaryotes with sexual reproduction resulting from meiosis.
def. Heredity: The genetic transmission of characteristics from parents to offspring.
def. Heterozygous trait: A trait with one allele of each kind.
def. Homologous chromosomes: A pair of chromosomes in a diploid cell containing similar genes at the corresponding loci throughout their length.
def. Homozygous trait: A trait with two identical alleles.
def. Monohybrid cross: A mating cross between two individuals that differ in ONE trait.
def. Nondisjunction: Abnormal meiotic division resulting in sex cells with too many or too few chromosomes.
def. Phenotype: The environmentally and genetically determined observable physical appearance of an organism, distinguished from its genetic constitution.
def. Principle of Independent Assortment (Mendel’s second law): When gametes are formed in diploid organisms during meiosis, each pair of homologous chromosomes have been sorted into gametes independently of how the other pairs of chromosomes were sorted.
def. Principle of Segregation (Mendel’s first law): When gametes are formed in diploid organisms during meiosis, the homologous alleles that control the trait separate from one another into different gametes.
def. Punnett square: A method used to determine the PROBABILITY of allele combinations in a zygote (offspring).
def. Recessive allele: An allele that does NOT express itself (does not show up) when a dominant homologous allele is present. (The dominant homologous allele masks the recessive allele.)
def. Sex chromosomes: Chromosomes determining the sex of an individual.
def. Trait: Any physical, physiological, and behavioral characteristic that an individual is born with.
10.1 INTRODUCTION
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• 10.11 Father of Genetics
Gregor Mendel. Proposed in 1866 basic laws of genetics based on pea plant experiments.
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• 10.12 Chromosomes
Humans have 46 chromosomes in their body cells (diploid number)
Humans have 23 chromosomes in their sex cells (haploid number)
def. Gene: portion of DNA that determines characteristics (traits) e.g. EAR LOBE SHAPE
def. Allele: alternative forms of a gene for a particular characteristic e.g. Free, Attached
def. Diploid organism: an organism having two sets of chromosomes – one from the maternal parent, the other from the paternal parent.
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• 10.13 Determination of Characteristics
• The type of characteristics (traits) is determined by:
1. the type of alleles received from your parents
-- chromosomes segregate during MEIOSIS
2. the interaction between these alleles.
3. the conditions the organism encounter (the environment)
10.2 SIMPLE GENETIC TRAITS -- controlled by two (2) alleles only
• 10.21 Terminology (monohybrid, one-trait inheritance)
-- Traits following “Simple Mendelian Rules” or “Mendelian Patterns of Inheritance”.
• 1. the type of alleles received from your parents
-- chromosomes segregate during MEIOSIS
• Offspring receive two alleles for each trait, one from each parent
The chromosome with these allele separate (segregate) during MEIOSIS when the gametes are produced. At fertilization the zygote (fertilized egg) receives two alleles for a particular trait (one allele from each parent -- one allele from the sperm, one allele from the egg).
def. Monohybrid cross (one-trait inheritance): A mating cross between two individuals that differ in ONE trait.
def. Dihybrid cross (two-trait inheritance): A mating cross between two individuals that differ in TWO traits.
def. Law of segregation: when gametes are formed by diploid organisms, the alleles that control the trait separate from one another into different gametes.
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• 2. the interaction between these alleles
• If the two alleles received from the parents are different there is a dominant/recessive relationship between them
def. Dominant allele: an allele that expresses itself and masks the effects of the other allele for the trait (the homologous allele).
def. Recessive allele: an allele that does not express itself (does not show up) when the homologous allele is present. The other allele (the homologous allele) the dominant allele, mask the recessive allele.
- - -
def. Genotype: the genetic constitution of an organism, distinguished from its physical appearance (the phenotype)
(= GENETIC CONSTITUTION)
def. Phenotype: the environmentally and genetically determined observable (outward) physical appearance of an organism, distinguished from its genetic constitution (the genotype)
(= PHYSICAL APPEARANCE)
- - -
def. Homozygous trait: both alleles for the trait are identical (pure breeding, “pure”): EE, ee
def. Heterozygous trait: alleles of one of each kind – one dominant & one recessive allele for a trait (“hybrid”): Ee
• 3. Conditions the organism encounter (the environment)
• The phenotype is also sometimes influenced by the environment (discussed later).
• 10.22 Natural characteristics (normal monohybrid traits)
-- Simple genetic traits are controlled by two (2) alleles only.
EXAMPLE: 1. Pea plant colors
EXAMPLE: 2. PTC taste
EXAMPLE: 2. Ear lobe shapes
Genotypes --- Phenotypes
EE --- Free ear lobes Homozygous dominant (“pure”)
Ee --- Free ear lobes Heterozygous (“hybrid”)
ee --- Attached ear lobes Homozygous recessive (“pure”)
• Predicting offspring
def. Monohybrid cross: mating cross between two individuals that differ in one trait.
def. Punnett square: A method used to determine the probability of allele combinations in a zygote.
def. Carrier: Any individual having a hidden, recessive gene.
Cross I: male , Free ear lobes EE X female , Attached ear lobes ee
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Cross II: male, free earlobes Ee X female, attached earlobes ee
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Cross III: male, free earlobes Ee X female, free earlobes Ee
• 10.23 Genetic disorders (simple monohybrid traits)
• Autosomal Recessive Disorders
EXAMPLES:
1. Phenylketonuria PKU
-- PKU gene located on chromosome 12
-- lack an enzyme to digest the AA Phenylalanine
‡ toxic breakdown product Phenylketone (damage the brain)
‡ accumulates in the urine
‡ avoid the artificial sweetener NutraSweet™ (cont. Phynylalanine)
-- Carriers (need both recessive allele to get the disease)
PP, Pp, pp pp -- get the disease, Pp -- carrier, PP -- normal
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2. Cystic Fibrosis
3. Tay-Sachs Disease
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• Autosomal Dominant Disorders
EXAMPLE:
1. Huntington Disease
-- Huntington Disease gene located on chromosome 4
-- degenerative nervous system disease (because of an exitotoxin) that leads to progressive degeneration of the brain cells, which in turn causes severe muscle spasms and personality disorders.
-- No carriers (all with the dominant allele get the disease)
HH, Hh, hh hh -- normal, Hh -- get the disease, HH - get the disease
def. Carrier: Any individual having a hidden, recessive gene.
• Lethal Dominant Gene
-- deadly --- eliminated?
-- late acting gene --- affect a persons after they have reproduced
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2. Neurofibromatosis
10.3 COMPLEX GENETIC TRAITS -- usually controlled by three or more alleles only
• 10.31 Complex gene patterns
• Incomplete Dominance
EXAMPLE: 1. Snapdragon color
F1 generation appearance is in between the two parental phenotypes.
EXAMPLE: 2. Hair-shape (Curly, Wavy, &. Straight)
F1 generation appearance is in between the two parental phenotypes.
EXAMPLE: 3. Sickle-cell anemia (SS, Ss, ss)
def. Incomplete Dominance: Inheritance in which F1 hybrids have an appearance intermediate between the two
parental phenotypes.
• Multiple alleles (Codominance)
Characteristics (traits) are determined by three or more alleles,
but only two present in each individual.
EXAMPLE: ABO blood types (3 alleles: IA, IB, i)
A-blood: ___________________, B-blood: ___________________
AB-blood: _________________, 0-blood: ___________________
• Pleiotropy (Single gene multiple effects)
One gene has more than one effect on the body.
EXAMPLE: Sickle-cell anemia (the shape of hemoglobin in the RBC)
def. Pleiotropy: Inheritance when one gene has more than one effect on the body.
1) less oxygen uptake -- negative effect on survival
2) resistance to malaria -- positive effect on survival
• Epistasis interactions (Gene pair interactions)
A gene influence (stop) the expression (of alleles) of a different gene.
EXAMPLE: Human hair color
(melanin production (enzymes) different amount of melanin produced)
• Polygenic inheritance (Continuous Variation)
Characteristics (trait) determined by three or more alleles,
all present in each individual.
EXAMPLE: 1. human skin color (melanin production vary)
EXAMPLE: 2. human eye color (melanin production vary)
• 10.32 Complex chromosomal patterns
• Two kinds of chromosomes:
1. autosomes: not involved in sex determination (22 pairs)
2. sex chromosomes: involved in sex determination (1 pair)
Two types of Sex Chromosomes:
-- X chromosome
-- Y chromosome:
– Y is much shorter (less genes)
– Y has male determining genes
– genes for traits on the X chromosome are MISSING on Y
• Sex-linked (X-linked) Inheritance
• Sex determination
(Punnett Square)
Males: XY
Females: XX
X X
X XX XX
Y XY XY
• X-linked inheritance examples
(Males have only one X chromosome)
Males: XY
Females: XX
-- this changes the mechanism in regards to genes being
expressed or not (Dominant allele vs. Recessive allele)
-- In males a recessive allele will always be expressed
‡ no dominant allele on the homologous chromosome to mask it.
-- In females a recessive allele will only be expressed if both alleles are present
EXAMPLE: 1. Hemophilia
EXAMPLE: 2. Color blindness (red & green)
Genotypes Phenotypes
XC X chromosome, normal
Xc X chromosome, colorblind
Y Y chromosome, no color vision genes
Cross: male , colorblind XcY X female , normal vision, but carrier XCXc
XcY X XCXc
(Punnett Square)
XC Xc
Xc XCXc XcXc
Y XCY XcY
Phenotypes: 25% female, carrier 25% female, colorblind;
25% male, normal 25% male, colorblind
• Nondisjunction
-- If the chromosomes do not separate correctly during meiosis.
1. When # of autosomes change
EXAMPLE 1: Down Syndrome (three (3) of chromosome 21)
2. When # of sex chromosomes change
EXAMPLE 2: XXY Klinefelter Syndrome
EXAMPLE 3: XXX Metafemale
EXAMPLE 4: X0 Turners Syndrome
EXAMPLE 5: Y0 (not viable)
10.4 ENVIRONMENTAL INFLUENCES
-- the conditions the organism encounter (the environment)
(Repetition)
• The type of characteristics (traits) is determined by:
1. the type of alleles received from your parents
2. the interaction between these alleles
3. the conditions the organism encounter (the environment)
• Identical twins >>> have identical genotypes (from the same zygote)
EXAMPLE: 1. freckles – sunlight
EXAMPLE: 2. diabetes mellitus – diet
EXAMPLE: 3. coats of some rabbits – temperature
EXAMPLE: 4. leave shape of some aquatic plants – water
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-- Disclaimer: "Dr. Nilsson's CyberOffice", at the time of writing located as a file under the South Texas Community College's (STCC) web server with the general URL http://stcc.cc.tx.us/, is the intellectual property of Dr. Jan A. Nilsson, member of STCC biology faculty. The content of Dr. Nilsson's CyberOffice does not necessarily reflect the opinions and beliefs of the STCC faculty, staff, administration, and Board of Trustees.