Patterns of Gene Inheritance
Jan 17, 2016
Patterns of Gene Inheritance
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
Gregor Mendel
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.
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?
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
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?
Gene locus
Defs
Example--Widow’s Peak
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.
One-Trait Crosses
GENOTYPE?GENOTYPE?
GENOTYPE?
Punnett SquareEXAMPLE 1—A Monohybrid Cross
Monohybrid cross
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).
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?
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
Segregation and Independent Assortment
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
HYPOTHETICAL EXAMPLE OF INDEPENDENT ASSORTMENT
Gene for
browneyes
Eye color
Gene for
blueeyes
Gene for
black hair
r
Gene for red hair
Hair color
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
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
What are the possible genotypes of this individuals gametes?
Dihybrid cross
Probabilities
Two-trait Testcross-How?
Genetic Disorders• Pedigree charts- show patterns of inheritance
Autosomal Dominant Disorders
• Autosomes---Non-sex chromosomes
• Dominant allelewhen under what genotype(s)will an
individual show the disorder?
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
Huntington disease
Autosomal recessive pedigree chart
• Autosomes---Non-sex chromosomes
• Recessive allelewhen under what genotype(s)will an
individual show the disorder?
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
Cystic fibrosis therapy
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.
• Polygenic (Quantitative) Traits
• Governed by more than one gene pair.
• Several genes determine the phenotype.
• Produce bell-shaped curve.
• EX: Skin color
Polygenic Inheritance
EX 2 Polygenic Inheritance :
Polygenic Disorders
• Examples: cancer, schizophrenia, hypertension, diabetes, etc– several genes involved– also environmental influences.
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
Inheritance of blood type
Details
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.
Incomplete dominance
Example 2-Incomplete Dominance
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?
Sickle-Cell Disease
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!!!
Everything’s Relative
WHAT’S HAPPENING??
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
REMEMBER!!!• Relate genetics to meiosis
• For genetics problems—first try to figure the genotype(s) of the gametes