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Ch.14 Patterns of Inheritance
Early Ideas & Assumptions about Heredity
Assumption 1 – Heredity occurs between species.
Assumption 2 – Traits are transmitted directly.
Darwin, in 1868 suggested that traits were blend in the offspring from the parents.
Josef Koelreuter, German botanist, carried out the first successful hybridizations in 1760
by crossing different stains of tobacco. He found that the traits of he was studying was
masked in one generation, only to reappear in the next. He found that this pattern
contradicted the direct transmission and also that traits are not blended.
T. A. Knight , an Englishmen gentleman farmer crossed two true-breeds of garden peas
(purple & white). He too was observing the same results that Josef Koelreuter had
seen.
Another century passed before the process of gene segregation was fully appreciated
because good records were not kept, nor were they quantified.
Gregor Mendel was an Austrian monk, born in 1822 - died 1884. He was educated at
the University of Vienna. He was the one who studied how traits were passed on and
how they were segregated.
He chose the Garden Pea for many reasons:
1. Earlier investigators had produced hybrid peas by crossing different varieties
2. A large number of true-breeding varieties of peas were available.
3. Pea plants are small and easy to grow several generations in a single year –
obtains results quickly.
4. Sexual organs of the pea are enclosed within the flower. Male and female parts
are in the same flower.
Flowers can self fertilize or one can cross-fertilize (remove the male parts prior to plants
fertilization). Mendel was careful to focus on only a few specific traits. Mendel
conducted his experiments in 3 stages:
1. He allowed pea plants of a given trait to produce progeny by self- fertilization. He
would breed them until only that trait was passed, regardless of the number of
generations.
2. Mendel then performed crossed between varieties exhibiting alternative forms of
traits.
3. Mendel permitted the hybrid offspring produced by these crosses to self-pollinate
for several generations. He was allowing alternative forms of a trait to segregate
among the progeny. He counted the numbers of offspring of each type in each
succeeding generation.
Mendel’s model of Heredity – has 5 elements
1. Parents do not transmit physiological traits directly to their offspring, but they
transmit information about traits called “factors”
2. Each individual receives 2 factors that may code for the same form or 2
alternative forms of the trait. (haploid/gamete)
3. Not all copies of a factor are identical. These factors are called ALLELES now
days. They can be the same HOMOZYGOUS or different HETEROZYGOUS.
The traits for the alleles are called GENES and they have a location on the
chromosome which is referred to as a LOCI.
4. The 2 Alleles (male & female) do not influence each other.
5. The presence of an allele does not insure that its form will be expressed by the
individual. The dominant allele is expressed over the recessive one. The totality
of the alleles are called GENOTYPE where as what their physical appearance is,
is called PHENOTYPE.
Mendel’s First law of Heredity or Law of Segregation = Alternative allele of a trait
segregate form each other in heterozygous individuals and remain distinct.
Mendel’s Second law of Heredity or Law of Independence = Genes located on different
chromosomes assort independently during meiosis.
Incomplete Dominance – Is when there is not a dominant nor recessive trait. The F1
offspring shared both features of the parent, as in the Japanese “four o’clocks”. One is
parent is red and other white, but their offspring is pink.
Sex linkage – a trait determined by a gene on the sex chromosome is said to be sexlinked. Male has XY and female has XX.
The ABO blood typing – the genes that code for blood types are CODOMINANT. This
means that each allele has its own effect and is neither dominant nor recessive.
A type can be either
B type can be either
AB type has to be
O type has to be
iA iA or iA i
iB iB or iB i
iA iB
i i
(AA, AO)
(BB, BO)
(AB)
(OO)
(The “i” stands for isoagglutinogen enzyme)
Blood types determined by the types of sugars added to lipids on the red blood cell
membrane. These glycolipids serve as surface antigens (recognition molecules) for
immune system. There is an Rh factor with the blood also which come from surface cell
markers. The immune system is based on a white blood cells ability to recognize the
antigens on red blood cell membranes. If you receive an antigen that is different from
yours, your white blood cells will produce antibodies (marker molecules telling other
white blood cells to phagocytically devour the tagged cell) in response. Your immune
system fights all invading cells with this recognition system.
Pleiotropy – when genes have multiple phenotypic effects
Epistasis – a gene at one locus alters the phenotypic expression of a gene at a second
locus.
Human Chromosomes and Genetic Abnormalities
Humans have 22 pairs of Autosomes (chromosomes that control traits other than sex).
The Y chromosome has few functional genes. The only “working” genes control
maleness. Any cell without a Y will be a female. Females have two X chromosomes.
One X will become inactive after sex is determined. If the female is heterozygous for a
characteristic, some cells will show one allele in their phenotype while other cells will
express the other allele. Which X chromosome to become inactive is purely random
from cell to cell.
Down’s Syndrome – monosomics – missing one member of a chromosome pair.
Trisomics – possessing an extra chromosome of a particular pair very few monosomics
or trisomics live.
Trisomy 21 = Down’s syndrome. Usually due to nondisjunction of the 21st chromosome
during meiosis in the production of the egg cell.
Symptoms = short, “squat”, round faces, large tongue and retarded mental
development. First described by J. Langdon Down in 1866. 1/750 children is born
Down Syndrome. 97% of all Down cases occur because of nondisjunction. 37% of the
cases occur because of translocation of a segment of chromosome 21 to another
chromosome.
Nondisjunction in sex chromosomes –
Klinefelter syndrome – nondisjunction of X chromosome – gamete is XXY produces a
male that is sterile, immature with reduced mental capacity.
XXX = 1 functional X and two inactive “Barr bodies”. Sterile, normal female
OY = enviable zygote. Cells can’t survive without the genes on the X
chromosome
Turner syndrome = OX chromosome make up. Short, webbed neck, will remain
immature sexually (sterile) with low I.Q.
XYY = nondisjunction in sperm formation.
GENETIC disorders
Cystic Fibrosis – defective gene causes mutated protein channel for chloride
transport. Thick mucus builds up in lungs that causes breathing difficulties and
infections in lungs. Recessive allele. Homozygous recessive individual usually dies by
25.
Sickle Cell Anemia – recessive gene causes production of faulty hemoglobin
(protein molecule that carries oxygen) in red blood cells. Low oxygen content in blood
causes RBC’s to take on sickle shape and not carry oxygen. Sickle cells often stick to
the walls of blood vessels causing clots. Homozygous sickle cell causes death as
young adult. Heterozygous condition – mild symptoms & highest cases in African
Americans
Tay-Sachs disease – defective recessive gene lets lipids build up in lysosomes in
brain cells. Lysosomes eventually burst and enzymes destroy cells. Newborn appears
normal, but within first year baby is blind. Rarely live past 5 years. Tay-Sachs genet
highest in descendents of Eastern and central European Jews.
Hemophilia – inability to produce a clotting protein in blood. There are 12 clotting
proteins. All must be produced in order for blood to clot. One of these proteins is
produced by a gene on the X chromosome. A heterozygous female will be normal. A
male with one recessive gene will therefore be a hemophiliac.
Phenylketonuria – recessive gene causes production of faulty enzyme.
Individual can’t break down the amino acid phenylalanine. Harmful chemicals result
from abnormal break down of phenylalanine. Destroy developing brain cells resulting in
retardation. If detected early enough diet can be altered as a preventative measure.
Huntingtons disease – dominant gene that causes brain deterioration and death
in an individual with only one allele for the disorder. Symptoms don’t present until the
individual is in late 30’s (after most have passed allele to children)