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Ch 14: Mendel and the Gene idea and Ch 15: The Chromosomal Basics of…
Ch 14: Mendel and the Gene idea and Ch 15: The Chromosomal Basics of Inheritance
Mendel's Model
Useful Genetic Vocab
character is an observable heritable feature that may vary among individuals, ex hair color
trait is one of two or more detectable variants in a genetic character, like having red or brown hair
homozygous is having two or more identical alleles for a given gene
homozygous dominant ex is AA, homozygous recessive is aa
heterozygous is having 2 different alleles for a given gene, ex Aa
phenotype is the observable physical and physiological traits of an organism
genotype is the genetic makeup or set of alleles of an organism
Law of Segregation/ Assortment
law of segregation occurs in anaphase I
law of segregation states that during the formation of gametes, the 2 alleles for a heritable character separate, so that each gamete receives only one allele
in Mendel's experiment the “heritable factor” for the recessive trait (white flowers) had not been destroyed, deleted, or “blended” in the generation but was merely masked by the presence of the factor for purple flowers, which is the dominant trait
law of independent assortment states that two or more genes assort independently, each pair of alleles segregates independently of any other pair of alleles, during gamete formation
law of independent assortment occurs in metaphase I
Punnet Square
punnet squares is a diagram used to show the predicted genotypic results of random fertilization in genetic crosses between individuals of known genotype
monohybrid cross is a cross between two organisms that are heterozygous for character being followed
dihybrid cross is a cross between 2 organisms that are each heterozygous for both of the characters being followed
Inheritance Patterns
Degrees of Dominance
complete dominance is the situation in which the phenotype of the heterozygote and dominant heterozygote are indistinguishable
incomplete dominance is a situation in which the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele
codominance is the situation in which the phenotypes of both alleles are exhibited in the heterozygote because both alleles affect the phenotype in separate, distinguishable ways
Relationship Between Dominance and Phenotype
an allele is called dominant because it is seen in the phenotype, not because it somehow subdues a recessive allele
Tay-Sachs disease is a human genetic disease caused by a recessive allele for a dysfunctional enzyme, leading to accumulation of certain lipids in the brain
only children who inherit two copies of the Tay-Sachs allele (homozygotes) have the disease
when a dominant allele coexists with a recessive allele in a heterozygote, they do not actually interact at all, it is in the pathway from genotype to phenotype that dominance and recessiveness come into play
Multiple Alleles
the ABO blood groups in humans, for instance, are determined by the two alleles a person has of the blood group gene; the three possible alleles are I^A, I^B, and i, a person’s blood group may be one of four types: A, B, AB, or O
pleiotropy is the ability of a single gene to have multiple effects
epistasis is a type of gene interaction in which the phenotypic expression of one gene alters that of another independently inherited gene
Sex Linked Genes
Basis of Sex
XX means female and XY means male
the X-Y system is in mammals, X-0 system is in insects, the Z-W system is in birds and fishes, and the haploid-diploid system is in bees and ants
if a sperm cell bearing an X chromosome fertilizes an egg, the zygote is XX, a female; if a sperm cell containing a Y chromosome fertilizes an egg, the zygote is XY, a male
X-Linked Genes
sex-linked genes is a gene located on either sex chromosome
most sex-linked genes are on the X chromosome
x-linked genes are located on the X chromosome and Y linked genes are located on the Y chromosome
mothers can pass X-linked alleles to both sons and daughters, as shown in for the inheritance of a mild X-linked disorder, red-green color blindness
in mammalian testes and ovaries, the two sex chromosomes segregate during meiosis
Sex-Linked Disorders
men cannot be carriers of x- linked genes but women can
males and females inherit a different number of X chromosomes leads to a pattern of inheritance different from that produced by genes located on autosomes
Duchenne muscular dystrophy is a human genetic disease caused by a sex-linked recessive allele; characterized by progressive weakening and a loss of muscle tissue
researchers have traced the disorder to the absence of a key muscle protein called dystrophin and have mapped the gene for this protein to a specific locus on the X chromosome
hemophilia is an X-linked human genetic disorder that is defined by the absence of one or more of the proteins required for blood clotting
Linked Genes
Genetic Recombination and Linkage
linked genes are genes located close enough together on a chromosome that they tend to be inherited together
genetic recombination refers to the production of offspring with combinations of traits that differ from those found in either parent
meiosis and random fertilization generate genetic variation among offspring of sexually reproducing organisms due to independent assortment of chromosomes, crossing over in meiosis I, and the possibility of any sperm fertilizing any egg
Crossing Over
parental types are offsprings with a phenotype that matches one of the true-breeding parental phenotypes
recombination types are offspring whose phenotype differ from that of the true breeding parental phenotypes
crossing over is the reciprocal exchange of genetic material between nonsister chromatids during prophase I of meiosis
Abnormal Chromosomes
nondisjunction is an error made in meiosis or mitosis in which members of a pair of homologous chromosomes fail to separate correctly
aneuploidy is a chromosomal aberration in which one or more chromosomes are present in extra copies or are defiant in number
monosomic refers to a diploid cell that only has one copy of a particular chromosome instead of the normal two
trisomic refers to a diploid cell that has 3 copies of a particular chromosome instead of the normal 2
polyploidy is a chromosomal alteration in which the organism possesses more than two complete chromosome sets. It is the result of an accident of cell division
Alterations of Chromosome Structure
a deletion occurs when a chromosomal fragment is lost, and is missing a certain gene
duplication is when a broken fragment may become reattached as an extra segment to a sister or nonsister chromatid
inversion is when a chromosomal fragment may also reattach to the original chromosome but in the reverse orientation
translocation is when a fragment joins a nonhomologous chromosome