NOTES FOR BIOLOGY 1001
SECTION 005
Spring 2005
DR. STEVEN POMARICO
CHAPTER 12
HUMAN GENETICS
The genetics for humans is difficult to work out:
-No test crosses
-Usually few offspring
-Long generation time
-Varied environmental effects
>>>>>>The chromosomal basis of sex produces unique patterns of inheritance
Sex is a phenotypic character determined by inherited chromosomes.
Humans usually have 44 autosomes (22 pairs) + the sex chromosomes
—Sex chromosomes are the chromosomes that determine the primary sex characteristics.
=> X-Y: Drosophila, humans, other mammals
-sex determined by whether male gamete (sperm) bears X or Y
-for the somatic cells the female is XX and the male is XY
The best way to study human genetics is by constructing a pedigree (See fig 12.9)
Dominant traits
-appear every generation
-appear in males and females
-if a child has the trait then at least one parent has the trait
Recessive traits
-skips generations
-only homozygous recessives exhibit the trait
-carriers (heterozygous) carry the gene hidden
-if a child has the trait both parents are “at least” carriers
The chance of two individuals carrying a given trait increases if the individuals are related. The closer the relationship the greater the chance.
That’s why cousins can’t marry.
>>>SINGLE - GENE INHERITANCE
Many traits are inherited in simple Mendelian fashion
-freckles
-long eyelashes
-ability to roll the tongue
The normal allele is usually dominant, while the recessive allele is usually the mutated gene.
Why is this the case?
If the defective gene is recessive, then the individual has a normal phenotype.
The defective gene persists.
However, if the defective gene was dominant, then the individual would have an impaired phenotype.
In this scenario the defective gene would quickly disappear from the population.
>>>SEX-LINKED INHERITANCE (See fig 12.12)
-traits unrelated to sex are on the sex chromosomes
-in humans, "sex-linked" usually means "X chromosome linked"
-X chromosome is bigger and therefore more genes possible
-most X chromosomes have no homologous loci on the Y chromosome
(i.e., can’t be homozygous or heterozygous at these loci)
-Fathers pass X to daughters
-Mothers pass X to sons and daughters
>>>Changes in chromosomal number: Aneuploidy and polyploidy
Errors in chromosomal number results from a nondisjunction during meiosis (I or II) or mitosis (See fig. 12.17).
In normal diploid cells there are two sets of chromosomes
Nondisjunction, can result in too many or too few chromosomes. Most nondisjunctions are fatal at the stage of the fetus.
Changes in chromosomal number
---Nondisjunction is a failure of choromosomes or chromatids to separate properly.
---Aneuploidy is the result of a nondisjunction in which one extra chromosome is present.
-results from the fusion of aberrant gamete with a normal gamete
-Trisomy => 2N + 1 -- Down's syndrome "Trisomy 21"
-Monosomy => 2N - 1
-Mitosis transmits the anomaly
---Polyploidy is the result of a nondisjunction in which an extra set of chromosomes is present.
-Triploidy (3N)
-Tetraploidy (4N)
-Polyploidy is common in plants
Trisomy 21, Down syndrome (See fig 12.18)
-most common trisomy 1/900
-percentage increases with an increase in mothers age over 40
(See fig. 12,19)
>Genetic disorder due to abnormal numbers of sex chromosomes
Turner Syndrome (XO) 1/5000
Trisomy X (XXX) 1/1000
Klinefelter Syndrome (XXY) 1/1000
“Supermales” (XYY) 1/1000
The mistakes are in meiosis and can be either in the production of sperm or eggs