1. Chapter 30
Heredity

2. Genetics The study of the mechanism of heredity
Nuclei of all human cells (except gametes) contain 46 chromosomes
Sex chromosomes determine the genetic sex (XX = female, XY = male)
Karyotype – the diploid chromosomal complement displayed in homologous pairs
Genome – genetic (DNA) makeup represents two sets of genetic instructions – one maternal and the other paternal

3. Alleles Matched genes at the same locus on homologous chromosomes
Homozygous – two alleles controlling a single trait are the same
Heterozygous – the two alleles for a trait are different
Dominant – an allele masks or suppresses the expression of its partner
Recessive – the allele that is masked or suppressed

4. Genotype and Phenotype
Genotype – the genetic makeup
Phenotype – the way one’s genotype is expressed

5. Segregation and Independent Assortment
Chromosomes are randomly distributed to daughter cells
Members of the allele pair for each trait are segregated during meiosis
Alleles on different pairs of homologous chromosomes are distributed independently
The number of different types of gametes can be calculated by this formula:
2n, where n is the number of homologous pairs
In a man’s testes, the number of gamete types that can be produced based on independent assortment is 223, which equals 8.5 million possibilities

6. Segregation and Independent Assortment
Figure 30.2

7. Crossover Homologous chromosomes synapse in meiosis I
One chromosome segment exchanges positions with its homologous maternal counterpart
Crossing over occurs, forming a chiasma
Genetic information is exchanged between homologous chromosomes
Two recombinant chromosomes are formed

8. Crossover
Figure

9. Crossover
Figure

10. Random Fertilization
A single egg is fertilized by a single sperm in a random manner
Considering independent assortment and random fertilization, an offspring represents one out of 72 trillion (8.5 million  8.5 million) zygote possibilities

11. Dominant-Recessive Inheritance
Reflects the interaction of dominant and recessive alleles
Punnett square – diagram used to predict the probability of having a certain type of offspring with a particular genotype and phenotype
Example: probability of different offspring from mating two heterozygous parents
T = tongue roller and t = cannot roll tongue

12. Dominant-Recessive Inheritance
Figure 30.4

13. Dominant and Recessive Traits
Examples of dominant disorders: achondroplasia (type of dwarfism) and Huntington’s disease
Examples of recessive conditions: albinism, cystic fibrosis, and Tay-Sachs disease
Carriers – heterozygotes who do not express a trait but can pass it own to their offspring

14. Incomplete Dominance
Heterozygous individuals have a phenotype intermediate between homozygous dominant and homozygous recessive
Sickling is a human example when aberrant hemoglobin (Hb) is made from the recessive allele (s)
SS = normal Hb is made
Ss = sickle-cell trait (both aberrant and normal Hb is made)
ss = sickle-cell anemia (only aberrant Hb is made)

15. Multiple-Allele Inheritance
Genes that exhibit more than two alternate alleles
ABO blood grouping is an example
Three alleles (IA, IB, i) determine the ABO blood type in humans
IA and IB are codominant (both are expressed if present), and i is recessive

16. Sex-Linked Inheritance
Inherited traits determined by genes on the sex chromosomes
X chromosomes bear over 2500 genes; Y carries about 15
X-linked genes are:
Found only on the X chromosome
Typically passed from mothers to sons
Never masked or damped in males since there is no Y counterpart

17. Polygene Inheritance
Depends on several different gene pairs at different loci acting in tandem
Results in continuous phenotypic variation between two extremes
Examples: skin color, eye color, and height

18. Polygenic Inheritance of Skin Color
Alleles for dark skin (ABC) are incompletely dominant over those for light skin (abc)
The first generation offspring each have three “units” of darkness (intermediate pigmentation)
The second generation offspring have a wide variation in possible pigmentations
Figure 30.5

19. Environmental Influence on Gene Expression
Phenocopies – environmentally produced phenotypes that mimic mutations
Environmental factors can influence genetic expression after birth
Poor nutrition can effect brain growth, body development, and height
Childhood hormonal deficits can lead to abnormal skeletal growth

20. Genomic Imprinting
The same allele can have different effects depending upon the source parent
Deletions in chromosome 15 result in:
Prader-Willi syndrome if inherited from the father
Angelman syndrome if inherited from the mother
During gametogenesis, certain genes are methylated and tagged as either maternal or paternal
Developing embryos “read” these tags and express one version or the other

21. Genomic Imprinting
Figure 30.6

22. Extrachromosomal (Mitochondrial) Inheritance
Some genes are in the mitochondria
All mitochondrial genes are transmitted by the mother
Unusual muscle disorders and neurological problems have been linked to these genes

23. Genetic Screening, Counseling, and Therapy
Newborn infants are screened for a number of genetic disorders: congenital hip dysplasia, imperforate anus, and PKU
Genetic screening alerts new parents that treatment may be necessary for the well-being of their infant
Example: a woman pregnant for the first time at age 35 may want to know if her baby has trisomy-21 (Down syndrome)

24. Genetic Screening, Counseling, and Therapy
Figure 30.7

25. Carrier Recognition
Identification for the heterozygote state for a given trait
Two major avenues are used to identify carriers: pedigrees and blood tests
Pedigrees trace a particular genetic trait through several generations; helps to predict the future
Blood tests and DNA probes can detect the presence of unexpressed recessive genes
Sickling, Tay-Sachs, and cystic fibrosis genes can be identified by such tests

26. Fetal Testing Is used when there is a known risk of a genetic disorder
Amniocentesis – amniotic fluid is withdrawn after the 14th week and sloughed fetal cells are examined for genetic abnormalities
Chorionic villi sampling (CVS) – chorionic villi are sampled and karyotyped for genetic abnormalities

27. Fetal Testing
Figure 30.9

28. Human Gene Therapy
Genetic engineering has the potential to replace a defective gene
Defective cells can be infected with a genetically engineered virus containing a functional gene
The patient’s cells can be directly injected with “corrected” DNA