patterns of gene inheritance. genetics is the study of genes what is a gene? a. a factor that...
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Patterns of Gene Inheritance
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Genetics is the study of genes
What is a gene?
A. A factor that controls a heritable characteristic
B. Something on a chromosome
C. Information stored in a segment of DNA
D. Something that encodes a protein
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Gregor Mendel
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LOTS O’ DEFINITIONS
• Gene---A piece of DNA that encodes a particular trait. EX a gene for eye-color
• Allele—an alternate form of a gene. EX—allele for blue eyes and allele for brown eyes
• Phenotype—the physical expression of a gene or allele. Ex. Blue eyes
• Genotype—the genetic composition of an individual.
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MORE DEFINITIONS• Locus—the location of a gene on a
chromosome. Plural=loci
• Dominant allele—expressed whether alone or in pairs. Symbolized by a capital letter. EX Brown eye allele=B
• Recessive allele—expressed only in the absence of a dominant allele. Symbolized by lowercase. EX blue eye allele=b
• What is the maximum # of alleles a diploid individual can have at any given locus?
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STILL MORE DEFINITIONS!!!
• Genotype—the genetic composition of an individual– Homozygous----containing a pair of the same
alleles. Can be
• Homozygous recessive—two recessive alleles EX. bb, or
• Homozygous dominant– two dominant alleles EX BB
– Heterozygous—containing two different alleles. EX Bb
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Law of Segregation
• Each individual has two factors (called genes today) for each trait.
• Factors segregate during gametogenesis. Ploidy level of gametes?
• Fertilization gives each new individual two factors again.
• Ploidy level after fertilization?
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Gene locus
Defs
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Example--Widow’s Peak
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Gametogenesis
• Homologous pairs separate during meiosis. When? a gamete has only one allele from each pair
of alleles. If the primary spermatocyte is genotype Ww,
the sperm cell would contain either a W or a w, but not both.
• Ww represents the genotype of an individual.
• Gametes are represented by W or w.
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One-Trait Crosses
GENOTYPE?GENOTYPE?
GENOTYPE?
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Punnett SquareEXAMPLE 1—A Monohybrid Cross
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Monohybrid cross
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The One-Trait Testcross
QUESTION: How can we determine the QUESTION: How can we determine the genotype of a dominant phenotype?genotype of a dominant phenotype?
Testcross: cross the dominant phenotype (unknown genotype) with the recessive phenotype (known genotype).
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The Law of Independent Assortment
• Law of Segregation involves 1 character. What about 2 (or more) characters?
• Each pair of alleles segregates independently of the other pairs
• All possible combinations of alleles can occur in the gametes
• EX: two pairs of alleles segregate independently of each other.
• When will different alleles not sort independently?
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E
n
e
N
Father
Mother
N
eE
n
e
n
E
N
e
n
E
N
e
N
E
n
Telophase II
Replication
Independent Assortment
N
E
n
e e
n
E
N
OR
Alignment of Homologs at Metaphase I
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Segregation and Independent Assortment
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MEIOSIS & GENETIC VARIATION
Independent Assortment
• Diploid organisms can produce 2n diff. gametes
• Ea. homologous pair can orient in two different ways
• Humans: 223 = 8,388,608
(est. 8.4 million)
Each homologous pair can orient in either of two ways
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HYPOTHETICAL EXAMPLE OF INDEPENDENT ASSORTMENT
Gene for
browneyes
Eye color
Gene for
blueeyes
Gene for
black hair
r
Gene for red hair
Hair color
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INDEPENDENT ASSORTMENT
OR
During meiosis I, tetrads can line up 2n different ways.
Brown eyesBlack hair
Blue eyesRed hair
Brown eyesRed hair
Blue eyesBlack hair
Meiosis I & II
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E
n
e
N
Father
Mother
N
eE
n
e
N
E
n
Replication
Linked Genes and Independent Assortment
N
E
n
E e
n
e
N
OR
Alignment of Homologs at Metaphase I
L l
l
lLL
L L ll
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What are the possible genotypes of this individuals gametes?
Dihybrid cross
Probabilities
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Two-trait Testcross-How?
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Genetic Disorders• Pedigree charts- show patterns of inheritance
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Autosomal Dominant Disorders
• Autosomes---Non-sex chromosomes
• Dominant allelewhen under what genotype(s)will an
individual show the disorder?
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Examples: Autosomal Dominant Disorder
• Neurofibromatosis– Small benign tumors, – Gene on chromosome 17.
• Huntington Disease– Progressive degeneration of the nervous
system– Normal until middle age– No treatment– Gene has been isolated (chromosome 4),
genetic testing can diagnose
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Huntington disease
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Autosomal recessive pedigree chart
• Autosomes---Non-sex chromosomes
• Recessive allelewhen under what genotype(s)will an
individual show the disorder?
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Autosomal Recessive Disorders
• Tay-Sachs Disease– Common among United States Jews of central and
eastern European descent. – Death by the age of three or four. – Defective enzyme in lysosomes.
• Cystic Fibrosis– Most common lethal genetic disorder among
Caucasians– Defective chloride ion transport protein – Osmotic imbalance results in thick mucous in lungs
and pancreatic ducts
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Cystic fibrosis therapy
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Phenylketonuria (PKU)• allele on chromosome 12.
• lack an enzyme needed for metabolism of phenylalanine (an amino acid)
• Urine test diagnostic.
• Brain damage unless controlled by diet.
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• Polygenic (Quantitative) Traits
• Governed by more than one gene pair.
• Several genes determine the phenotype.
• Produce bell-shaped curve.
• EX: Skin color
Polygenic Inheritance
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EX 2 Polygenic Inheritance :
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Polygenic Disorders
• Examples: cancer, schizophrenia, hypertension, diabetes, etc– several genes involved– also environmental influences.
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Multiple Allelic Traits
• Often more than two alleles exist for a particular gene locus.
• Maximum # of alleles at any given locus?
• Each individual inherits only two alleles for these genes!!!
• EX: Human Blood type
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Inheritance of blood type
Details
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Incompletely Dominant Traits• Codominance--both alleles equally
expressed in a heterozygote. EX ABO Blood type
• Incomplete dominance--heterozygote shows intermediate phenotype, representing a blending of traits. EX Wavy hair in Caucasians
• Phenotypic ratio ?• 1 : 2 : 1.
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Incomplete dominance
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Example 2-Incomplete Dominance
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Sickle-Cell Disease• Caused by incompletely dominant alleles.• Single nucleotide mutation causes abnormal
hemoglobin.• HbA = normal hemoglobin• HbS = sickled condition.• Genotype of normal?• Genotype of sickled homozygote?• Genotype of intermediate phenotype?
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Sickle-Cell Disease
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Molecular Explanation-Dominance
• Simple Dominance– One-half the amount of gene product sufficient for
phenotype
• Incomplete Dominance– Recessive allele not expressed in heterozygote– Homozygous dominant –2 doses, full phenotype– Heterozygote—1 dose, reduced phenotype
• Codominant– Both alleles expressed, combined phenotype
Let’s Get Small!!!
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Everything’s Relative
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WHAT’S HAPPENING??
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SUMMARY• Physical Basis of Genes• Lots o’ Definitions• Law of Segregation• Law of Independent Assortment• One-Trait Crosses• Law of Independent Assortment• Two-Trait Crosses• Genetic disorders• Polygenic Traits• Incomplete Dominance/Codominance
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REMEMBER!!!• Relate genetics to meiosis
• For genetics problems—first try to figure the genotype(s) of the gametes