Module 6

Module 6—Mendelian Genetics: The Transmission of Traits to the Next Generation

Module Glossary

 

Consult the glossary in the textbook for other definitions that you may need to complete your work.

 

addition: to add or sum

 

autosomal: refers to a trait that is due to a gene on one of the first 22 pairs of chromosomes—not the sex chromosomes; autosomal traits occur equally in males and females

 

autosomal: a condition is autosomal if the pedigree shows no significant difference in the number of each gender affected by the condition

 

autosomal dominant: these conditions appear in each generation in the pedigree, and cannot arise from two parents who did not have the condition

 

autosomal recessive: these conditions often skip a generation in a pedigree, and are indicated if two parents without the condition produce a child with the condition

 

autosomal recessive: a condition caused by a gene found on an autosome (non-sex chromosome), which only occurs in those with the homozygous recessive genotype

 

biodiversity: the variation present in the genes, species, and ecosystems of a biome; at the species level refers to the variety of alleles, genotypes, and phenotypes present for each gene; used as an indicator of the biological health of a species, ecosystem, or biome

 

chromosomal theory of inheritance: the theory that chromosomes are strings of genes and that each gene has a specific location on a specific chromosome

co-dominance: similar to incomplete dominance—neither allele is dominant; in the heterozygote, both distinct phenotypes appear—not a mixture, e.g., red and white cow colour alleles produce red and white hairs in the heterozygote; in blood types the AB heterozygote makes both A antigens and B antigens—not a mixture

 

continuous traits: traits that have a range of phenotypes, such as human height or eye colour with its many shades; is due to the polygenic effect of many genes together

 

contrasting characteristic: a trait that has two alleles that are expressed in a different way (e.g., the trait for height in pea plants can have the contrasting characteristics of tall or short)

 

crossover percentage: same as recombination frequency

 

dominant: if two alleles are present, the allele that is expressed is the dominant one (e.g., if an individual has both tall and short alleles, but is tall, then tall is dominant over short)

 

environment: all non-genetic factors that affect a person’s characteristics; e.g., how he or she was raised, light levels, friends, temperature, good and bad experiences, diet and nutrients available, and so on

 

epistasis: a type of polygenic inheritance where two genes collectively determine a phenotype of a trait; e.g., the B gene codes for type of colour (B = black, b = brown) and the C gene determines whether colour occurs at all (C = colour, c = no colour); three phenotypes result—white, brown, and black

 

expression: refers to the transcription and translation of the gene to produce the requested protein

 

Whether or not an allele will be expressed is affected by whether or not the allele is dominant; whether the product is needed by the cell; the stage of the life cycle; and, to some degree, the environmental conditions. In undifferentiated cells, all genes are expressed; in differentiated cells only some genes are expressed.

 

F1 generation: offspring of the P1 generation; F stands for filial generation—meaning brothers and sisters

 

F2 generation: offspring produced as a result of individuals of the F1 generation mating with other individuals of the F1 generation

 

factor: the term used by Gregor Mendel to explain inheritance of a trait; today we use the term gene

 

filial: the offspring of a cross

 

genotype: what the genes say; two symbols that indicate which alleles the individual has for a particular trait (e.g., Tt)

 

hemophilia: an X-linked recessive condition; caused by the recessive allele of the Factor VIII gene on the X chromosome; characterized by inability to clot blood

 

heterozygous: a genotype where the alleles are different (e.g.,Tt)

 

homozygous (true breeding): a genotype where both alleles are the same (e.g. ,TT is homozygous dominant, tt is homozygous recessive)

 

hybrid: the offspring of two parents with different inheritable traits; often termed heterozygous

 

incomplete dominance: some genes have two alleles—neither of which are dominant; in the heterozygote the phenotype is a mixture of both alleles; e.g., if the white and black alleles are incompletely dominant, the heterozygote will be grey

 

inheritable trait: a characteristic that is determined by genes, not the environment; a trait that is capable of being passed from parent to offspring

 

innate: a characteristic one is born with

 

law of independent assortment: during meiosis, any allele from one trait can end up in the same gamete with any allele from another trait; e.g., an individual with the DdGg genotype can produce DG, Dg, dG, or dg gametes; only applies to genes that are not linked on the same chromosome

 

law of segregation: the two genes that an organism has for each trait are separated during meiosis so that only one gene can enter a gamete; explains why body cells are diploid, but gametes are haploid

 

linked genes: genes that exist on the same chromosome; e.g., if the acne gene and belly fat gene are on the same chromosome, then they are linked; with A and B linked together and a and b linked together, or vice versa

 

malaria: a disease of warm, wet areas caused by the plasmodiumprotist found in the Anopheles mosquito

 

When the mosquito bites a person, the plasmodium enters the blood stream and lays its eggs in red blood cells. Sickled red blood cells typical of those people with sickle cell anemia (SCA) are harder for the plasmodium to attack, so those with SCA are less likely to die of malaria. The homozygous recessive condition (ss) confers a disadvantage (SCA) and an advantage (protection from malaria).

 

map distance: how far apart (in centimorgans) two genes are on a chromosome; the recombination frequency from a linkage cross is used as the map distance between two genes; e.g., 44% of the offspring of the linkage cross were recombinants, therefore the two genes are 44 map units or 44 cM apart.

 

map unit: the unit used is the centimorgan (cM); because the recombination frequency is a percentage, the largest map distance is 100 cM

 

monohybrid: an organism that is heterozygous for one trait e.g., Tt

 

monohybrid cross: a cross of two individuals, each of which is heterozygous for one trait (e.g., Tt X Tt; always produces a 1:2:1 genotypic ratio and 3 dominant to 1 recessive phenotypic ratio)

 

multiple alleles: some genes have more than two alleles; an order of dominance is given to determine phenotypes from genotypes; e.g., in a particular flower, colour alleles are red, amber, yellow, and albino in order of decreasing dominance

 

nature: in reference to the nature versus nurture debate, refers to the components of an individual’s phenotypes that are determined by his or her genes; e.g., approximately 50% of intelligence is thought to be due to nature

 

non-parental type: in a linkage cross, any offspring of the above cross that don’t look like the parents (Aabb or aaBb) are non-parental types, are the results of crossing over, and are recombinants

 

nurture: refers to the component of one’s phenotypes that are determined by one’s environment

 

P1 generation: the first set of parents; parents of the F1 generation

 

parental type: in a linkage cross, if the offspring look like either parent, then they are parental types; e.g., if we assume A and B are linked together and a and b are linked together in the linkage cross AaBb X aabb, any offspring that are AaBb or aabb are parental types

 

pedigree: a chart outlining the generations and relationships within a family line

 

pedigree: a picture of a family history of a genetic condition that indicates generation, gender, birth order in the generation, and parental and offspring relationships; the presence of the condition is indicated by a blackened-in symbol

 

phenotype: the observable characteristic of the organism e.g., tall height or short height; is determined by genotype e.g., the Tt genotype results in a tall phenotype

 

phenotypic ratio: a proportion expressing all the possible visible physical traits in a generation

 

pleiotropy: the reverse of polygenic inheritance; where one gene affects the phenotypes for several traits; e.g., the PKU gene affects mental retardation, skin colour, hair colour, and other traits

 

polygenic inheritance: where more than one gene (often several) are involved in determining the phenotype for one characteristic; e.g., eye colour is actually the result of multiple genes that collectively contribute to the final eye colour

 

probability: the likelihood of an event occurring; can be expressed as a decimal (0.20), fraction (), or percentage

 

product: to multiply

 

progeny:  offspring

 

pure: an individual with both alleles for a trait being dominant, or recessive

 

recessive: if two alleles are present, the allele that is hidden is recessive

 

recombination: during crossing over in prophase I of meiosis, genes from non-sister chromatids trade places, making new allele combinations in the gametes; e.g., if A is linked to B and a to b, then any gametes that are Ab or aB are the results of recombination or crossingover.

 

recombination frequency: the number of recombinant offspring divided by the total number of offspring X 100; expressed as a percentage; used as a map distance between the two genes; e.g., the recombination frequency was 32%, therefore the two genes are 32 map units apart on the chromosome

 

selective breeding: choosing stock because of its physical, behavioural, or functional characteristics

 

sex-linked: a trait that is found in one gender more than another; contrary to autosomal traits that are found equally in both genders

 

sickle cell anemia (SCA): an autosomal recessive disease that causes sickle-shaped red blood cells to form, which get caught in blood vessels, stopping blood flow to tissues

 

test cross: a cross done to determine the genotype of an individual with a dominant phenotype; the individual with the unknown genotype is mated with a homozygous recessive individual—if any offspring have the recessive phenotype, the unknown is a heterozygote

 

trait: a characteristic

 

trihybrid: a genotype that is heterozygous for three traits, for example AaBbCc; sometimes used in a general way to refer to genetics problems using three traits

 

X-linked: a trait that is found in one gender more than in the other gender, and so is sex-linked; contrairy to autosomal traits, which are found equally in both genders

 

X-linked dominant: a condition is X-linked dominant if the pedigree shows the condition is found in females significantly more than in males

 

X-linked recessive: a condition is X-linked recessive if the pedigree shows the condition is found in males significantly more than in females; carrier moms pass the allele to affected sons; affected sons cannot pass the allele to their sons, but pass it to their daughters, who in turn become carriers

 

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