Basics Of Genetics And Heredity

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Carries all the genetic information in most organisms

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Chromosomes

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The unique location on a chromosome where a particular gene is located

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Locus

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Forms of the same gene, which differ in their nucleotide sequence

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Alleles

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Number of alleles that a gene can have, resulting in variations of the trait

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Two or more

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A trait can be created by how many genes?

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A single gene or a combination of genes

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The combination of alleles that make a particular trait

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Genotype

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The expression of a trait in an organism

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Phenotype

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An allele that masks the effect of its partner allele

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Dominant gene

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An allele that is masked by its partner allele

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Recessive gene

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Mendel's first law which states that allele pairs separate or segregate during gamete formation, and randomly unite at fertilization

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Law of segregation

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Mendel's third law of inheritance, which states that alleles can be dominant or recessive, and the dominant allele, if present, will always determine the trait

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Law of Dominance

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Mendel's second law of inheritance, which states that allele pairs separate independently during the formation of gametes, which means that traits are transmitted to offspring independently of one another

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Law of Independent Assortment

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Sometimes ________ are present for a trait and the alleles may combine in different ways, such as with blood type

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more than two choices

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Traits that are inherited together

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Linked traits

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When pieces of chromosomes break off during meiosis and recombine, increasing variation among the species

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Crossing over

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Traits whose loci are on the sex chromosomes

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Sex-limited traits

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Combination of chromosomes that make an organism female

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XX

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Combination of chromosomes that make an organism male

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XY

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What is required for a female to exhibit a recessive X-linked trait

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Copies of the same recessive gene inherited from the X chromosome of both parents

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Recessive X-linked traits are expressed more commonly in which gender?

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Males

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What are common examples of recessive X-linked traits?

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Color-blindness, hemophilia

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Why are Y-linked traits not much of an issue?

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The Y chromosome is small and does not contain many genes, therefore few traits are Y-linked, making Y-linked diseases rare.

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Autosomal traits that are expressed in males who have only one recessive gene, but which can only be expressed in females if they possess two recessive genes

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Sex-influenced traits (such as baldness)

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Traits that are produced from the expression of more than one gene (as is true with most traits)

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Polygenic traits

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A diagram that is used to predict the outcome of a particular cross or breeding experiment

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Punnett Square

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Who devised the approach of the Punnett Square?

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Reginald C. Punnett

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A notation that allows easy prediction of the result of a genetic cross

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Punnett Square

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A cross between two individuals where only one trait (controlled by a single gene) is considered

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Monohybrid cross

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When both alleles for a given gene are the same in an individual, that individual is ________ for the trait

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Homozygous

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When the two alleles for a given gene are different in an individual, the individual is ________ for that trait

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Heterozygous

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A cross between two individuals where two separate traits are considered

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Dihybrid cross

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When a trait has no genes that are dominant and offspring produce a mix of the parents

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Incomplete dominance

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In a Punnett Square, dominant and recessive traits are notated in this fashion

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Dominant - upper case letters
Recessive - lower case letters

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In a Punnett Square, incomplete dominance uses this notation

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Superscript letters

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In the three different alleles that dictate blood type for humans, type ___ and type ___ blood are both dominant alleles, while type ___ blood is recessive

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A and B blood types are dominant
O blood type is recessive

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When chromosomes do not properly separate during meiosis and both members of one homologous chromosome pair migrate to the same pole

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Nondisjunction

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Result of nondisjunction in meiosis

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Gaemetes with missing or extra chromosomes, almost always leading to genetic defects

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Result of a gamete possessing an extra number 21 chromosome fusing with a normal gamete

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Downs syndrome, causing metal retardation, heart defects, respiratory problems, and deformities in external features

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Result of nondijunction of sex chromosomes (sperm has XY or O, or egg has XX or O)

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Female who has only one X chromosome and is sterile, with female sexual characteristics that are generally underdeveloped

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Primary types of chromosomal abnormalities

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Missing chromosome
Extra chromosome
Deletion (portions deleted)
Duplication
Translocation
Inversion (reverse orientation)

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X and Y chromosomes

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Sex chromosomes

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Chromosomes other than X and Y

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Autosomes

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An autosomal recessive genetic defect that results in an individual with abnormal hemoglobin, who cannot effectively transport oxygen through the body

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Sickle-cell anemia

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Inability to code for a clotting factor required to form normal blood clots

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Hemophilia

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When one gene effects the phenotypic expression of a second gene, such as occurs often with pigmentation

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Epistasis

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What happens when a blood transfusion is made between individuals of incompatible blood types?

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The immune system attacks the new red blood cells as a foreign substance, resulting in agglutination, or clumping of the blood, and possibly death

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The reason why males always express traits from their X chromosome, regardless of whether the allele is recessive or dominant

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Because there is no similar gene delivered by their Y chromosome

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A highly condensed, inactive X chromosome found in female mammals

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Barr body

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When one of the two X chromosomes for a female embryo does not uncoil into chromatin

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X-inactivation

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X-inactivation in female mammals means that all of their cells are not functionally identical because ________

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Early in embryonic development in females, one of the two X chromosomes is randomly and permanently inactivated in nearly all somatic cells (but not always the same one)

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X-activation often compensates for sex-linked genetic defects because _____

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At least some of the cells will not express the defect, as in the case of hemophilia where some cells still secrete the necessary clotting factor

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When both inherited alleles are completely expressed (as is the case with blood types)

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Codominance

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In a Punnett Square, what do P, F₁, and F₂ refer to?

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Generations - parent, familial generation 1, and familial generation 2

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Three kinds of inheritance

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Complete dominance
Incomplete dominance
Codominance

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When one gene effects the phenotypic expression of another gene (as often occurs with pigmentation)

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Epistasis

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When a single gene has more than one type of phenotypic expression (as is the case with sickle cell anemia)

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Pleiotropy

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Before setting up the x- and y-axes of a Punnett Square for a dihybrid cross, you need to figure out what?

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Determine the gene combinations that each parent could produce

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Traits that are not one thing or another, but are expressed within a range, like height (very short --> very tall)

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Continuous variation

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Continuous variation is usually a result of this, also known as the interaction of many genes shaping a single phenotype

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Polygenic inheritance

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Polygenic inheritance is the opposite of ___________

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Pleiotropy

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Genes that reside on the same chromosome and so cannot segregate independently (meaning, they are usually inherited together)

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Linked genes

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The greater the distance between two genes on one chromosome, the higher the possibility that _________ may affect the genes during synapsis

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Crossing over

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Maps of the relative position of genes on a given chromosome

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Cytological map

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A DNA sequence that increases the rate of transcription of nearby genes

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An enhancer

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What percentage of genes are typically expressed in the average human cell?

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Around 5%

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A stretch of DNA that can move from one location to another in the genome

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Transposable element

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What happens if a transposable element moves into the middle of a gene?

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The gene becomes nonfunctional

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A process that moves DNA from one place in the genome to another

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Chromsomal rearrangement

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Method of regulating gene expression

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Chromatin condensation
Enhancers

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A condition in which a cell has lost or gained one or more chromosomes

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Aneuploidy

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A condition in which one chromosome is missing

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Monosomy

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A condition in which there are three copies of a chromosome

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Trisomy

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A technique for testing the genotype of an embryo or fetus in the uterus

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Amniocentesis

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Who discovered sex-linked traits by studying the Drosophila fly?

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Thomas Hunt Morgan (1910)