Heredity (11)

Asexual Reproduction

One parent, identical offsprings

Nucleotide

The monomer of DNA

Codon

Three nucleotides that code for one amino acid

Chromosome

Continuous strand of DNA

Homologous Chromosomes

Chromosomes with the same genes but different alleles

Allosomes

Sex chromosomes

Haploid (N)

Having one set of chromosomes

Polyploid

Having more than two sets of chromosomes; more commonly seen in plants than in animals

Meiosis

Halves the number of chromosomes; produces gametes as four haploid daughter cells in animals

Prophase I

Homologous chromosomes pair up, recombination occurs, and synaptonemal complex dissolves

How do homologous chromosomes pair up in prophase I?

They are held together by a protein structure called the synaptonemal complex

Anaphase I

Homologous chromosomes separate and are pulled to opposite poles; sister chromatids remain attached at their centromeres

Oogenesis

One primary oocyte (2N) becomes one ovum (N) and three polar bodies

How are the polar bodies created?

The primary and secondary oocytes are not divided evenly during cytokinesis; polar bodies have significantly less cytoplasm and cannot be fertilized

Dominant Allele

An allele which is always expressed when present

Law of Segregation

Only one of the two gene copies present in an organism is distributed to each gamete

Incomplete Dominance

Two alleles produce an intermediate phenotype in a heterozygote, rather than one being fully dominant

Polygenic Inheritance

One trait is controlled by multiple genes, leading to a bell curve of phenotypes

Sex Linkage

Genes are carried on sex chromosomes, leading to different patterns of inheritance and expression between sexes

Sex (X) Linked Recessive Inheritance

Both parents → all offspring; mother → son; daughter → father; male bias

What asexual reproduction is carried out by prokaryotes?

Binary fission

What asexual reproduction is carried out by eukaryotes?

Mitosis; fragmentation or budding

Sexual Reproduction

Two parents, different offspring

What sexual reproduction is carried out by prokaryotes?

Bacterial conjugation using plasmids; not sexual reproduction but a form of gene transfer

What sexual reproduction is carried out by eukaryotes?

Meiosis and fertilization

What is the main advantage of sexual reproduction?

Sexual reproduction increases genetic diversity

What is the main disadvantage of asexual reproduction?

Negative mutations are accumulated in later generations

Genome

The entire genetic makeup of an organism

Gene

The basic unit of inheritance at a point on a chromosome

Allele

Variant of a gene

Trait

Physical characteristic of an organism, may be the result of genes and alleles

Autosomes

Chromosomes that are not sex chromosomes

Diploid (2N)

Having two sets of chromosomes

What is the basic pathway of fertilization?

Haploid gametes (sperm and egg) fuse to form a diploid zygote (fertilized egg)

Which polyploid sets are fertile, and why?

Even-numbered sets; they can divide evenly during cell division

Alternation of Generations

The sexual reproduction process that occurs in vascular plants, fungi, and mosses; mitosis creates gametes while meiosis creates spores

Why do chromosomes occur in pairs?

Each chromosome is inherited from each parent

What is pollen?

A male gametophyte that produces the sperm

What can mosses tell us about the evolution of diploid organisms?

Organisms began with a dominant haploid reproductive phase which evolved to become a dominant diploid reproductive phase, until the haploid phase disappeared

Where did meiosis likely evolve from?

Mitosis

What happens in meiosis I?

Halving of chromosomes and genetic diversity

How does recombination (crossing over) occur?

Homologous chromosomes swap segments of DNA at the chiasma

Chiasma

The point where homologous chromosomes crossover in prophase I

How are homologous chromosomes held together for metaphase I after the synaptonemal complex dissolves?

They are held together by the chiasmata

Metaphase I

Homologous chromosomes align along the metaphase plate, each pair of homologous chromosomes attaches to a different centrosome, and independent assortment occurs

Independent Assortment

Chromosomes align along the metaphase plate randomly, increasing genetic diversity

Telophase I

Chromosomes decondense and new nuclei form

Spermatogenesis

One primary spermatocyte (2N) becomes four spermatids (N) which are then modified to become sperm

Gene Linkage

Genes that are close together on a chromosome are very unlikely to be separated by recombination

Nondisjunction

Failure to separate chromosomes in anaphase I or chromatids in anaphase II, leading to cells with extra or missing chromosomes

How do sex chromosomes segregate in meiosis?

Homologous chromoses are helpd together by chiasma

Do female (XX) chromosomes encounter problems during meiosis?

No, the two X chromosomes are homologous and can pair together

How do male (XY) chromosomes overcome problems with pairing during meiosis?

The X and Y chromosomes pair on the pseudoautosomal region

Pseudoautosomal Region

A region on both the X and Y chromosomes that are homologous

Do Y chromosomes experience any genetic diversity?

No, all the genes on the Y chromosome are permanently linked and do not exchange

What did Gregor Mendel discover?

Invisible factors in the form of genes/chromosomes and genetic inheritance through pea plant experiments

What did Frederick Griffith discover?

DNA carries genetic information

Law of Dominance

Some alleles are dominant, and some are recessive

Recessive Allele

An allele which is only expressed in the absence of a dominant allele

Genotype

Genetic characteristics

Phenotype

Physical characteristics

Homozygous Dominant

Two dominant alleles

Heterozygous

One dominant and one recessive allele

Homozygous Recessive

Two recessive alleles

How did Mendel know the genotypes of P1?

Pea plants are true-breeding plants that only produce matching offspring when self-pollinated

Law of Independent Assortment

The segregation of alleles for one gene occurs independently to that of any other gene

Why are incomplete dominance and codominance not Mendelian Genetics?

It breaks the Law of Dominance

Codominance

Two alleles may be expressed simultaneously in a heterozygote, rather than one being fully dominant

Pleiotropy

One gene has an effect on multiple traits

Lethal Alleles

Homozygous alleles that are lethal in development and are never expressed

Epistasis

One gene masks the effects of another gene; can be dominant or recessive

Complementary Genes

A special type of epistasis in which one gene masks and copies the effects of another gene; occurs when both genes are needed to determine the final phenotype

Maternal Effects

Phenotype is determined by the mother's genotype

Mitochondrial and Chloroplast Inheritance

They have their own DNA but they inherited exclusively from the mother

Why aren't all mitochondria and chloroplasts identical in a cell?

They don't have a mechanism to ensure all daughter cells have the exact same genetic makeup during cell division

Penetrance

Proportion of individuals with a genotype that expesses the associated phenotype

Complete Penetrance

The trait is expressed 100% of the time

Incomplete Penetrance

The trait is expressed less than 100% of the time

Autosomal Dominant Inheritance

Offspring → parent; cannot skip generations

Autosomal Recessive Inheritance

Both parents → all offspring

Sex (X) Linked Dominant Inheritance

Offspring → at least one parents; father → all daughters; son → mother; cannot skip generations; female bias

Sex (Y) Linked Inheritance

Father → all sons; !father → !son

Mitochondrial Inheritance

Mother → all offspring; !mother → !all offspring