PATTERNS OF GENE INHERITANCE
I.
TERMINOLOGY
A. Genes are sections of chromosomes that code for protein
1. They are represented by letters in a particular sequence and at particular spots (loci)
on a homologous pair of chromosomes.
B. Phenotype
1. An organism's inherited physical characteristics
2. Determined by an organisms genotype
C. Genotype
1. An organism's genetic makeup
2. Represented by two letters, each representing an allele on homologous chromosomes
3. May be represented by a descriptive phrase
a) Homozygotes
(1) Have two of the same alleles for a gene
(a) May be homozygous dominant or homozygous recessive
b) Heterozygotes
(1) Have two different alleles for a gene
D. Homologous chromosomes
1. Diploid organisms have two sets of chromosomes; 1 inherited from both parents
2. Chromosomes with the same genes (but possible different alleles) are referred to as
homologous chromosomes
E. Alleles
1. Alleles represent alternative forms of a particular gene
a) They have the same position on homologous chromosomes and affect the same
trait
2. Dominant allele
a) If present, a person will show the encoded phenotype
b) Represented by a capital letter
3. Recessive allele
a) Is masked by a dominant allele
(1) Trait is only shown if an organism has two recessive alleles
b) Represented by a lower case letter
4. Codominant alleles
a) An allele will be expressed regardless of the other allele
(1) Example: A and B alleles will both be expressed to determine blood type
5. Incomplete dominance
a) Dominant trait is only fully expressed with two dominant alleles
b) Recessive trait is only expressed with two recessive alleles
c) Heterozygotes will have a phenotype halfway between the dominant and
recessive traits
II. MENDEL'S LAWS
A. Mendel noted that many traits in diploid organisms are determined by 2 genes
1.
One of the factors could be dominant (tall) over the other, which is recessive (short)
2.
The Law of Segregation
a) Each organism contains 2 genes for each trait and these genes segregate during
meiosis
b) Each gamete (haploid) contains only one gene from each pair
c) Fertilization gives rise to a new individual (diploid) with 2 genes for each trait
III. MONOHYBRID CROSSES
A. Gametes (haploid) contain only one allele per gene due to meiosis
B. Punnett square
1.
Determine the type of gametes parents can form and place at the sides of a Punnett
square
2. Determine the genotypes of children that can result from random fertilization
C. Probability
1. The probability that 2 or more independent events will occur together is the product
(multiplication) of their chances occurring separately
2. If both parent's genotypes are Ww, chance of obtaining either W or w in the gametes
is 1/2
a) The chance of forming a WW = 1/2 x 1/2 = 1/4
b) The chance of forming a Ww = 1/2 x 1/2 = 1/4
c) The chance of forming a wW = 1/2 x 1/2 = 1/4
(1) By summing all 3 possibilities, the chance of an offspring with a dominant trait is 3/4, and the
chance of an offspring with a recessive trait is 1/4
D. Testcross
1. Used to determine is someone with a dominant trait is homozygous dominant or
heterozygous
2. An individual with the dominant phenotype (either homozygous or heterozygous) is
crossed with an individual having the recessive phenotype
3. By examining the offspring ratio, one can determine if the person with the dominant
phenotype is homozygous or heterozygous
IV. DIHYBRID CROSSES
A. Independent assortment of alleles
1. Each pair of alleles segregates (assorts) independently of the other pairs and that all
possible combinations of genes can occur in the gametes
a) This is due to crossing over in prophase I and random alignment of homologous
chromosomes on the equator during anaphase I
B. Punnet square
1. Genotypes of parents are represented with 4 letters
a) There are 2 alleles for each trait
2. Genotypes of haploid gametes are represented with 2 letters
a) Gametes will contain one allele for each trait in every possible combination
3. All possible fertilizations are then calculated
C. Probability
1. If a dihybrid (WwSs) reproduces with another dihybrid (heterozygous for 2 traits), 4
possible gametes are formed with 16 possible offspring
a) Forms a 9:3:3:1 phenotypic ratio
2. If the monohybrid probability for widow's peak = 3/4 as well as for short fingers, then
the dihybrid probability of an individual having both widow's peak and short fingers =
3/4 x 3/4 = 9/16
V. SINGLE GENE DISORDERS
A. Autosomal Dominant Genetic Disorders
1. Caused by a dominant allele on an autosomal (nonsex) chromosome
2. Few lethal disorders of this type, since the effected persons would die, not passing the
allele to the next generation
3. Huntington Disease
a) Begins at middle age, so may be passed on to next generation
(1) Offspring has a 50:50 chance of inheriting gene from effected parent
b) Leads to insanity and death
B. Autosomal Recessive Genetic Disorders
1. Caused by recessive alleles on autosomal (non-sex) chromosomes
a) Child may inherit the disease from two normal heterozygote parents
b) Heterozygotes are carriers, and serve to pass the alleles to the next generation
2. Cystic Fibrosis
a) The most common lethal genetic disease among Caucasians in the United States
b) Due to the inability of chloride ions unable to pass through cell membrane channel
proteins
c) Without chloride, osmosis does not occur and the mucus becomes thick and
viscous which impedes respiration
3. Tay-Sachs Disease
a) Most common among Jewish people
b) Due to a lack of the enzyme hexosaminidase A
(1) This enzyme breaks down glycosphingolipids in lysosomes
c) Leads to accumulation of lysosomes, which result in neurological disorders
4. Phenylketonuria
a) Lack of an enzyme to break down the amino acid phenylalanine
b) Phenylalanine build up during development results in brain damage and mental
retardation
c) Treatable by limiting dietary intact of phenylalanine
C. Autosomal Incompletely Dominant Genetic Disorders
1. Neither allele is dominant over the other; but together, the phenotype is intermediate
between the 2
2. Sickle-cell anemia
a) The allele is selected for malaria-infested Africa because the malarial parasite
cannot survive within RBC with the sickle cell hemoglobin
3. Homozygous dominants and heterozygotes have normal phenotype
a) Heterozygotes are protected from malaria
4. Homozygous recessives have the sickle cell disease
VI. MULTIPLE ALLELES
A. Within a population, there may be 3 or more alleles that affect a particular trait;
however, diploid organisms can have only 2 of those alleles for that trait
B. ABO Blood Types
1. Blood types
a) A = blood type A antigen on surface of RBC
(1) Genotype may be AA or AO
b) B = type B antigen on RBC
(1) Genotype may be BB or BO
c) O = no antigens on RBC
(1) Genotype must be OO
d) AB blood type has alleles A and B
(1) Genotype must be AB
2. A and B alleles are codominant with each other, but both are dominant over the
recessive O allele
VII. SEX LINKED INHERITANCE
A. Sex chromosomes in humans are the X and Y chromosomes
1. Females are XX and males are XY
2. All factors being equal, there is a 50% chance of having a boy or a girl
a) Females must donate a X chromosome to their eggs, males may donate either the
X or Y to their sperm
B. Genes on the sex chromosome are termed sex linked
1. Most of the genes are found on the larger X chromosome and are called X-linked
genes
a) Males are more likely to show recessive X-linked traits, since they will only get one
allele for that trait
b) A female can only show recessive X-linked traits if her father showed the trait and
her mother was a carrier (or had the trait)
2. X-linked Recessive Disorders
a) Color blindness
b) Hemophilia
c) Muscular Dystrophy