CHAPTER 14/CHAPTER 15 GENE INHERITANCE

What is the difference between a gene and an allele?

**gene** = heritable factor; specific sequence of DNA that is responsible for the inheritance of a particular trait

**allele** = individual variants of any one gene

Distinguish between the following terms: genotype and phenotype; trait and character; dominant and recessive alleles, homozygous and heterozygous

**genotype** = the 46 chromosomes (and the genes they carry) in each of your somatic cells constitute your genotype

**phenotype** = result of the selective gene expression from the genotype constitutes your phenotype (physical appearance)

**character** = heritable feature that varies among individuals (such as flower color)

**trait** = each variant for a character such as a purple or white color for flowers

**dominant allele** = determines organism’s appearance regardless if it is homozygous dominant (RR) or heterozygous (Rr)

**recessive allele** = has no noticeable effect (effect is only noticeable when it is homozygous recessive or rr)

**homozygous** = if two chromosomes contain the same allele, they are homozygous

**heterozygous** = if two chromosomes have two different alleles they are heterzygous

What are the 4 hypotheses that came out of Mendel’s work with pea plants

1. alternative versions of genes account for variations in inherited characteristics; alternatives are called alleles
2. for each character, an organism inherits two alleles, one from each parent
3. if two alleles at a locus are different, the dominant allele determines the organism’s appearance; the recessive allele has no noticeable effect
4. law of segregation - the two alleles are separated during gamete formation and must end up in different gametes therefore each gamete receives only one copy

Understand the 2 laws that resulted from Mendel’s work: the law of segregation and independent assortment.

* **law of segregation**: the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes; egg or sperm only gets one of the two alleles present in the parent and the segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis
* **law of independent assortment:** each pair of alleles segregates independently of any pair of alleles during gamete formation and the exception is linked traits; the punnett square is simulating all possible ways that chromosomes could line up and separate in Meiosis 1

Explain how linked genes can be unlinked.

they can be unlinked through recombination (crossing-over) which involves the exchange of genetic material between homologous chromosomes during meiosis

if they are also far enough apart, they can be genetically unlinked

Explain recombination, including when it occurs, where, and why

* recombination is known as crossing over and it is the process where genetic material between homologous chromosomes is exchanged
* it occurs during prophase 1 of meiosis
* creates genetic diversity
* the further apart the alleles are, the greater chance of recombination

Explain how phenotypic expression in the heterozygote differs with complete dominance, incomplete dominance, and codominance

* **in complete dominance** = phenotype of the heterozygote is the same as the dominant homozygote and the recessive allele is not expressed in the phenotype
* **in incomplete dominance** = phenotype of the heterozygote is intermediate between the two homozygotes for example if one allele is red and the other is white, itll be pink
* **in codominance** = both alleles are expressed in the phenotype of the heterozygote, for example if one person has an allele for type A blood and the other has type B blood then the persons blood type will be AB which expresses both A and B antigens on the surface of their red blood cells

Define and give examples of pleiotropy, polygenic inheritance, and epistasis.

**pleiotropy** - when a single gene affects multiple traits (most alleles have multiple phenotypic effects); sickle cell anemia and cystic fibrosis are pleiotropy

**polygenic inheritance** - when two or more genes work additively to affect a phenotype; wide range of skin pigmentation is not determined by a single gene; height is an example as height is determined by the interaction of multiple genes each contributing a small effect to the overall phenotype

**epistasis** - expression of a gene at one locus alters the expression of a gene at a second different locus; albinism is epistasis

How does the expression of Dominant versus Recessively inherited disorders differ?

**dominant inherited disorders** require one copy of the mutated gene in order for the disorder to be expressed while **recessively inherited disorders** require two copies (one from each parent) in order for the gene to be expressed

What is a genetic carrier? Explain how one specific genetic carrier has a selective advantage against malaria.

* a genetic carrier is a parent that carries one copy of a recessive allele for a genetic disorder
* carriers can pass on the recessive allele to their offspring which increases their risk of the disorder appearing in future generations
* one specific genetic carrier with a selective advantage against malaria is individuals who carry the sickle cell trait; the individuals who inherit one copy of the sickle cell allele do not exhibit any symptoms of the disorder and have less severe malaria symptoms

Understand how large-scale chromosome alternations result from nondisjunction.

* nondisjunction is the failure to separate homologous chromosomes during meiosis resulting in gametes with abnormal numbers of chromosomes
* large-scale chromosomes alternations can occur when these abnormal gametes are involved in fertilization, resulting in zygotes with extra or missing chromosomes, such as trisomy 21 (down syndrome) or monosomy x (turner syndrome)

Is aneuploidy always fatal?

no, there are several combinations of aneuploidy that are compatible with life (trisomy 21)

What are sex-linked genes, and when are they hemizygous? How does their expression differ in males versus females?

* sex-linked genes are genes that are located on the sex chromosome (X or Y)
* hemizygous in males, only one copy of the gene (X)
* females have two copies of the X chromosomes and can either be homozygous or heterozygous for sex-linked genes)
* males show the phenotype of any x-linked gene they inherit since they do not have a second copy to mask it
* females need two copies of a recessive x-linked gene to exhibit the corresponding phenotype