Heredity

 

I.                    The Vocabulary of Genetics

A.     Chromosome=bar-like structures of tightly coiled chromatin, visible during cellular division.

B.     Homologous chromosomes=chromosomes that carry the same genes and determine the same traits.  In a human, there are 46 chromosomes forming 23 homologous pairs.

1.      Autosomes=of the 23 pairs of chromosomes present in human cells, 22 pairs of them have nothing to do with the sex of the individual.

2.      Sex chromosomes=of the 23 pairs of chromosomes present in human cells, 1 pair determines the sex of the individual.

C.     Gene=one of the biological units of heredity that determines the traits of an individual.

D.     Alleles=different versions or alternate forms of a gene (example: blue eyes, brown eyes, green eyes, etc).  Alleles are often designated as being dominant or recessive.

1.      Dominant alleles can mask the effects of other alleles (example:  widow’s peak). Theses alleles are always designated with a capital letter such as “A”.

2.      Recessive alleles are easily masked by the effects of other alleles (example: straight hairline).  These alleles are always designated with a lower case letter such as “a”.

E.      Genotype= a person’s combination of alleles.

1.      Homozygous dominant=the person possesses two dominant alleles (AA).

2.      Heterozygous=the person possesses one dominant allele and one recessive allele (Aa).

3.      Homozygous recessive=the person possesses two recessive alleles (aa).

F.      Phenotype=a person’s outward or physical expression of their allele combinations (genotype).

1.      A homozygous dominant person shows the dominant trait on the outside.

2.      A homozygous recessive person shows the recessive trait on the outside.

3.      A heterozygous person shows the dominant trait on the outside because the dominant trait masks the presence of the recessive.

 

II.                 Sexual Sources of Genetic Variation

A.     Mendel’s Law of Segregation

1.      Each organism contains two factors (alleles) for each trait and these randomly align along the metaphase plate.

2.      The factors then segregate during the formation of gametes so that each gamete contains only one factor for each trait. NOTE: nondisjunction can occur and lead to aneuploidy (missing one or more chromosomes) and polyploidy (adding one or more chromosomes)

3.      This reshuffling of the factors helps explain how variations come about and why offspring differ from their parents.

B.     Law of Independent Assortment

1.      Members of one pair of factors separate independently of members of another pair of factors.

2.      Therefore, all possible combinations of factors can occur in the gametes.

C.     Crossing over of Homologous resulting in gene recombination

1.      During Meiosis I (prophase I), two of the four chromatids (one maternal and the other paternal) may cross over at one or more points and exchange corresponding gene segments.

2.      The recombinant chromosomes contain new gene combinations, adding to the variability arising from independent assortment.

D.    Random Fertilization

1.      The third source of genetic variation is random fertilization of eggs by sperm.

2.      When just considering independent assortment and random fertilization, any resulting offspring represents ONE out of the close to 72 TRILLION zygotes possible.

 

III.               Types of Inheritance

A.     Dominant-Recessive Inheritance

1.      This type of inheritance is common with traits like hitchhiker thumb, rolling the tongue, hairline, PTC tasting, etc.

2.      In every case, the dominant trait masks the recessive.

3.      Punnett Squares are useful tools for determining genetic inheritance of dominant/recessive traits.

4.      However, Punnett Squares only predict the probability of having a certain percentage of offspring with a particular genotype or phenotype. 

5.      The larger the number of offspring, the more likely that the ratios will conform to the predictions.

6.   Examples of human traits: Dimples, Earlobes, Freckles, etc...

7.      Perform sample Punnett Squares!  Be sure you can predict probability in percentage as well as genotypic and phenotypic ratios.

B.     Incomplete Dominant Inheritance

1.      Traits are not masked in the heterozygous form.  Instead, an intermediate phenotype is produced.

2.      Example:          A       +       B        =         C

3.      Example:          Red flowers  +  White flowers  =  Pink flowers

4.      Sickle cell anemia is a human condition associated with this type of inheritance where the intermediate form has sickle cell trait but not full blown sickle cell disease.

5.   Other examples include wavy hair (intermediate of curly and straight hair) and nose and eye size

6.      Perform sample Punnett Squares!  Keep in mind that genotypic ratios are calculated the same way but the phenotypic ratio must reflect the possibility of a third phenotype.

C.     Codominant Inheritance and Multiple Alleles

1.      There are some traits that demonstrate more than two alleles.

2.      Blood type is a common multiple allele pattern of inheritance.

3.      Because two alleles are “dominant” neither has the ability to mask the other; instead they are Codominant and mixture of the two alleles shows up in the phenotype of the offspring,

4.      Example:        A      +       B        =         AB

5.      Example:   Red flower + White flower = Red flower with white dots.

6.   Human examples: AB blood type

7.      Perform sample Punnett Squares! Make sure you can determine probability and genotypic and phenotypic ratios.

D.    Sex-linked Inheritance

1.      All of the other patterns of inheritance mentioned above are demonstrations of genes carried on autosomal chromosomes and an individual has equal chances of getting the gene whether that person is male or female.

2.      Sex-linked inheritance however, demonstrates traits that are carried on the sex chromosomes and an individual’s chance of getting the trait varies with the sex of the individual.

3.      Most sex-linked traits are carried on the X chromosome while very few are carried on the Y chromosome.

4.      X-linked traits affect both males and females because both sexes will receive at least one X in their genotype (XX=females; XY=males). X-linked disorders are either X-linked dominant (Coffin-Lowry syndrome and Rett syndrome) or x-linked recessive (color blindness, hemophilia A&B, Hunter syndrome, Wiskott-Aldrich syndrome, and Duchenne muscular dystrophy)

5.      Y-linked traits only affect males because females do not receive a Y chromosome.

6.      Perform sample Punnett Squares!  Be sure you can predict probability and genotypic/phenotypic ratios.

E.     Polygenic Inheritance

1.      Polygenic inheritance is the result from several different genes at different locations within the genetic makeup work together to produce a particular phenotype.

2.      Skin color is based on three separate gene pairs.

3.      Other examples include hair color and height

F.      However, environment can affect the expression of genes.

1.      Maternal drug use can alter normal gene expression during embryonic development.

2.      Nutrition and diet

3.      Hormonal deficits and excesses

  

IV.              Nontraditional Inheritance

A.     Genomic Imprinting

1.      For every gene there is a maternal copy and a paternal copy (i.e., maternal allele and a paternal allele)

2.   In some cases one copy is "turned off" in a parent-of-origin

3.   Therefore, a gene can be "imprinted" and its expression is then dependent upon only one allele

4.   Imprinting is due to epigenitic modifications (e.g. changes in chromatin proteins) rather than DNA sequence changes

5.      It is reversible and occurs all over each generation.

6.   Disorder examples: Prader-Willi syndrome,  Beckwith-Wiedemann syndrome,  Silver-Russell syndrome, and Angelman syndrome

B.     Extrachromosomal Inheritance

1.      Cytoplasmic genes (from mitochondrial DNA or mtDNA) pass to offspring via the ovum and help to determine certain characteristics.

2.      Deletions of mutations in mitochondrial genes are responsible for some rare genetic diseases such as Leber's hereditary optic neuopathy and Kearns-Sayre syndrome

 

V.                 Genetic Screening, Counseling, and Therapy

A.     The likelihood of an individual carrying a deleterious recessive gene may be assessed by constructing a pedigree.

B.     Karyotypes can also be performed on fetal cells to determine the presence of extra, missing, or mutated chromosomes.

C.     During amniocentesis, a small amount of amniotic fluid is extracted for testing.  Physicians can diagnose some disorders from chemicals in the fluid itself, while other disorders may show up in tests performed on the cultured fetal cells present in the fluid.

D.     In chorionic villi sampling, a physician inserts a narrow tube through the cervix and suctions out a tiny sample of fetal tissue from the chorionic villi from the placenta.  The fetal tissue samples are used to conduct immediate Karyotypes.