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Chapter Notes for Lecture: E.N. Marieb, HUMAN ANATOMY & PHYSIOLOGY,5TH Edition, , Benjamine Cummings Publisher, 2001 Prepare from : V.A. Austin’s PowerPpoint Presentation (ISBN: 0-8053-5469-7), CD ROM: Pearson Education, Inc. , 2003.

 

Chapter 30

Heredity

 

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

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

Genotype and Phenotype

•      Genotype – the genetic makeup

•      Phenotype – the way one’s genotype is expressed

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

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

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

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

•      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

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)

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

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

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

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

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

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

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

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)

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

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

 

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