1. HEREDITY

2. THE VOCABULARY OF GENETICS
Introduction:
The nuclei of all human cells except gametes contain the diploid number of chromosomes (46) or 23 pairs of homologous chromosomes
Two of the 46 chromosomes, and the remaining 44 are 23 pairs of autosomes that guide the expression of most other traits
Homologous chromosomes are the two chromosomes that make up a pair in a diploid cell, one from the egg and one from the sperm
The karotype is the diploid chromosomal complement displayed in homologous pairs

3. PREPARING A HUMAN KARYOTYPE

4. THE VOCABULARY OF GENETICS
Chromosomes are paired, with one coming from each parent, and the genes on those chromosomes are also paired
Alleles are any two matched genes at the same locus (site) on homologous chromosomes
Homozygous means that the two alleles are the same
Heterozygous means the two alleles are different
A dominant allele is one that will mask or suppress the expression of the other allele
A recessive allele is the allele that is suppressed by the dominant allele and will only be expressed in the homozygous condition
Genotype and Phenotype
A person’s genetic makeup is called his or her genotype
The way the genotype is expressed in the body is that individual’s phenotype

5. SEXUAL SOURCES OF GENETIC VARIATION
During metaphase of meiosis I the alignment of the tetrads along the center of the cell is completely random, allowing for the random distribution of material and paternal chromosomes into the daughter cells
In meiosis I the two alleles determining each trait are segregated, or distributed to different gametes
Alleles on different pairs of homologous chromosomes are distributed independently of each other
Independent assortment of homologues during meiosis I can be calculated as 2n, where n is the number of homologous pairs
During meiosis I homologous chromosomes may exchange gene segments, a process called crossing over, which gives rise to recombinant chromosomes that have contributions from each parent
Random fertilization occurs because a single human egg is fertilized by a single human sperm in a completely haphazard basis

6. INDEPENDENT ASSORTMENT OF HOMOLOGOUS CHROMOSOMES DURING METAPHASE OF MEIOSIS I

7. CROSSING OVER AND GENETIC RECOMBINATION OCCURRING DURING MEIOSIS I

8. TYPES OF INHERITANCE Dominant-Recessive Inheritance
A Punnett square is used to determine the possible gene combinations resulting from the mating of parents of known genotypes
Dominant traits include widow’s peaks, dimples, freckles, and detached earlobes
Genetic disorders caused by dominant genes are uncommon because lethal dominant genes are always expressed and result in the death of the embryo, fetus, or child
Recessive traits include some desirable conditions such as normal vision, but they also include most genetic disorders
Some traits exhibit incomplete dominance, where the heterozygote has a phenotype intermediate between those of the homozygous dominant and the homozygous recessive
Some genes exhibit more than two allele forms, such as blood type, leading to a phenomenon called multiple-allele inheritance

9. PROBABILITIES OF GENOTYPES AND PHENOTYPES RESULTING FROM A MATING OF TWO HETEROZYGOUS PARENTS

10. TYPES OF INHERITANCE
Inherited traits determined by genes on the sex chromosomes are said to be sex-linked
The Y chromosome is much smaller than the X chromosome and lacks many of the genes present on the X that code for nonsexual characteristics, such as red-green color blindness
Genes found only on the X chromosome are said to be X-linked; those found only on the Y chromosome, such as those that code for maleness, are said to be Y-linked
Polygene inheritance results in continuous or quantitative phenotype variation between two extremes, and depends on several gene pairs at different locations acting in tandem (an example of this is skin color)

11. POLYGENE INHERITANCE OF SKIN COLOR BASED ON THREE SEPARATE GENE PAIRS

12. ENVIRONMENTAL FACTORS IN GENE EXPRESSION
In many situations environmental factors override or at least influence gene expression
Phenocopies are environmentally produced phenotypes that mimic conditions that may be caused by genetic mutations
Environmental factors may also influence genetic expression after birth, such as the effect of poor infant nutrition on brain growth, body development, and height

13. NONTRADITIONAL INHERITANCE
Genomic imprinting somehow tags genes during gametogenesis as either paternal or maternal and confers important functional differences in the resulting embryo
Disorders have been traced to errors in mitochondrial genes that are transmitted to the offspring almost exclusively by the mother

14. GENETIC SCREENING, COUNSELING, AND THERAPY
When one of the parents of a developing embryo displays a recessive disorder it is important to determine if the other partner is a heterozygote and thus a carrier for that trait
One way to identify if an individual may be a carrier for a particular trait is to do a pedigree, which traces a genetic trait through several generations
Blood tests, both simple and a more sophisticated blood chemistry test, and DNA probes can be used to detect the presence of unexpressed recessive genes

15. PEDIGREE

16. GENETIC SCREENING, COUNSELING, AND THERAPY
Fetal testing is used when there is a known risk of a genetic disorder
The most common type of fetal testing is amniocentesis, where a needle is inserted into the amniotic sac to withdraw amniotic fluid for testing
Chorionic villi sampling (CVS) suctions off bits of the chorionic villi from the placenta for examination
Human gene therapy and genetic engineering have the potential to alleviate or even cure diseases, especially those traced to one defective gene

17. FETAL TESTING