Heredity: Crash Course Biology #9

Heredity is the passing of genetic traits from one generation to the next.  For almost 2,000 years people believed that babies were a mixture of both parents’ traits.  Mom supplied the building blocks and Dad supplies the life force.  These ideas from Aristotle had not been challenged until the middle of the 19th century.  A century later, scientists started learning even about DNA and the way it worked.

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In the 1800s, Gregor Mendel, an Austrian monk conducted a series of experiments using garden pea plants to study heredity.  Unlike Aristotle’s ideas, Mendel’s findings provided a framework for understanding how traits are actually passed from parents to offspring.  With the work of scientists over time, we now know that DNA, in the form of chromosomes, are passed from parent to offspring.  A gene is a section of DNA on a chromosome that determines a trait.  Humans have 23 chromosomes.  Most of our traits are Mendelian traits, which means that they determined by more than one gene.  Very few of our traits are polygenic, or determined by one gene.  

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One example of a Mendelian Trait in humans is earwax consistency - wet or dry.  The gene located for this trait is on Chromosome 16.  When two heterosexual (male and female) parents engage in sexual intercourse, a haploid sperm cell from Dad’s semen will fertilize a haploid egg cell from Mom.  Because they are each haploid, these sex cells only contain half a set of DNA - 23 chromosomes.  Once joined together, a diploid zygote results.  This has a full set of 46 chromosomes, one half from Mom and one half from Dad, each with an allele for each gene for each trait.

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In this example, Mom has the wet earwax phenotype and Da has the dry earwax phenotype.  The allele for wet earwax is dominant whereas the allele for dry earwax is recessive.  Though the dominant allele masks the recessive allele, it does not mean that the recessive allele is less common than the dominant allele.  

In order to determine what Mom and Dad’s baby’s earwax consistency is, we have to look at their genotypes, not just their phenotype.  Because Mom inherited the allele for dry earwax from her mom and the allele for wet earwax from her dad, she is heterozygous for the earwax trait.  Because Dad inherited the allele for dry earwax from both his mom and ad, he is homozygous for the earwax trait.  

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Reginald C. Punnett invented the Punnet Square in order to diagram the possible outcomes of a particular genetic cross of two parents.  On the top, one parent’s genotype is written.  On the left, the other parent’s genotype is written.  Then, to show the segregation of alleles upon gamete formation, each of the two alleles is separated for each parent.  Afterward, each allele is dropped down to its respective box to illustrate possible outcomes of sexual reproduction - the combination of egg and sperm to form a diploid baby.  

In the earwax example, possible genotype outcomes are Ww and ww.  Genotype Ww is homozygous, meaning that this gives the phenotype wet earwax whereas ww is heterozygous recessive, meaning that it gives the phenotype dry earwax.  There is a ½ or 50% chance that each genotype and each phenotype will result.  

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Of the 23 chromosomes each of our cells has, 22 pairs are called autosomes and 1 pair is called sex chromosomes.  For this 23rd pair, females have two full length X chromosomes (XX) whereas males have 1 full lenth X chromosome and 1 shorter Y chromosome (XY).  

Balding is an example of a genetic trait that is linked to sex chromosomes.  Because females have 2 full length X chromosomes, they may have a dominant allele that will counteract a recessive allele.  On the other hand, males only have 1 full length X chromosome, meaning that a recessive alelle on this chromosome will not be able to be counteracted from any allele for a gene on the Y chromosome.  To be bald, men only need 1 recessive allele on their X chromosome - inherited from their mother.  Women are rarely bald because they would have to receive 2 recessive alleles, one on each of their X chromosomes.

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