Chapter 5 - Non-Mendelian Inheritance

Unit 2

1) Expression of genes in offspring directly influenced their traits

2) Genes are passed unaltered from generation to generation, except when rare mutations occur

3) Genes obey Mendel’s law of segregation

4) When crosses involve more than one gene, the genes obey Mendel’s law of independent assortment

Genes that have Mendelian inheritance patterns conform to which four rules?

Maternal Effect

An inheritance pattern for certain nuclear genes in which the genotype of the mother directly determines the phenotype of her offspring. Surprisingly, the genotypes of the father and offspring themselves do not affect the phenotype of the offspring.

This occurs due to the accumulation of gene products that the mother provides to her developing eggs.

Dominant / genotype / dextral / sinistral

Fill in the blank…

How the maternal effect was discovered…

Boycott was studying morphological features of the water snail, L.peregra. The shell and internal organs of this species can be arranged in one of two directions: dextral (right-handed) and sinistral (left-handed). The dextral orientation is ( ). When he performed multi-generation crosses, he found that the phenotype of the offspring depended solely on the ( ) of the mother. The Dd or DD mothers produced ( ) offspring while dd mothers produced ( ) offspring. The genotypes of the father nor the offspring affected the phenotype of the offspring.

Oogenesis / diploid / haploid / D / d / dextral / sinistral / cleavage plane

Fill in the blank…

The maternal effect phenomenon can be explained by ( ). Maturing oocytes are surrounded by maternal cells that provide them with nutrients. These nurse cells are ( ), while the oocyte becomes ( ). The snail’s shell curvature depends on the cleavage pattern of the egg immediately after fertilization. If a female is heterozygous for the maternal effect, the haploid oocyte can receive either the ( ) or ( ) allele gene products from the nurse cell.

The D gene products cause egg cleavage that promotes a ( ) shell while the d gene products cause egg cleavage that promotes a ( ) shell. Even if the sperm is carrying the D allele, the shell will still be sinistral because expression of the sperm’s gene would be too late to change early embryonic development.

The orientation of the ( ) in the earliest stages of development carries through to the adult.

Embryogenesis

Fill in the blank…

Maternal effect genes encode RNA and proteins that play important roles in the early steps of ( ), i.e. cell division, cleavage pattern, body axis orientation. Accumulation of maternal effect gene products before fertilization allows these steps to proceed very quickly after fertilization.

Epigenetic Inheritance

An inheritance patterns in which a modification occurs to a nuclear gene or chromosome that alters gene expression. However, the expression is not permanently changed over the course of many generations. It is reversible and does not change the DNA sequence.

It is caused by DNA (e.g. methylation) and chromosomal modifications that can occur during oogenesis, spermatogenesis, or early embryonic development.

Dosage Compensation

The purpose of this technique is to offset differences in the number of active sex chromosomes. It is a crucial genetic mechanism that ensures equal expression of genes located on sex chromosomes despite differences in the number of chromosomes between males and females. It has been studied extensively in mammals, Drosphilia, and C.elegans. Depending on the species, it occurs via different mechanisms.

Z / W / Z

Fill in the blank…

In birds, the sex chromosomes are ( ), a large chromosome containing many genes, and ( ), a much smaller chromosome containing few genes. Males are ZZ while females are ZW. It appears that the ( ) chromosome in males does not undergo a general mechanism of dosage compensation like one of the X chromosomes in female mammals. Some may be dosage-compensated but many are not.

Barr body / X

Fill in the blank…

Dosage compensation in mammals

Barr and Bertram identified a highly condensed structure in the interphase nuclei of somatic cells in female cats but not males. This structure was known as a ( ), proposed to be a highly condensed ( ) chromosome.

Chromosome condensation / Lyon

Fill in the blank…

In mammals, dosage compensation in mammals occurs by ( ), also called the ( ) hypothesis or X-chromosome inactivation.

X^b / X^B / inactivation

Fill in the blank…

An example of Lyon hypothesis….

A mouse with patches of black and white fur. A female mouse has inherited two X chromosomes. One from its mother that confers white coat color ( ) and one from its father that confers black coat color ( ). Random X chromosome ( ) results in the white and black patches of fur.

Compacted / somatic / active / Barr bodies

Fill in the blank…

During X-chromosome inactivation, the DNA becomes highly ( ) and most genes on the X chromosome can’t be expressed. When it is replicated during cell division, both copies remain highly compacted and inactive. This inactivation is passed along to all future ( ) cells. Researchers have found that mammalian cells can count their X chromosomes and allow only one of them to remain ( ).

Additional X chromosomes are converted to ( ).

Genomic Imprinting

A phenomenon in which a segment of DNA is marked and the effect is maintained throughout the life of the organism inheriting the marked DNA.

Monoallelic expression

Depending on how the genes are marked during genomic imprinting, the offspring expresses either the maternally-inherited or paternally-inherited allele, not both.

1) Establishment of the imprint during gametogenesis

2) Maintenance of the imprint during embryogenesis and in the adult somatic cells

3) Erasure and reestablishment of the imprint in the germ cells.

Imprinting in terms of genetic imprinting is an epigenetic process that can be divided into what three stages?

Extranuclear Inheritance

Inheritance patterns involving genetic material outside the nucleus. The two most important examples are due to genetic material within organelles like mitochondria and chloroplasts. This term is also called cytoplasmic inheritance.

Nucleoid / circular / chloroplasts

Fill in the blank…

The genetic material of mitochondria and chloroplasts is located in a region called the ( ), in which the genome is composed of a single ( ) chromosome containing double-stranded DNA. A nucleoid can contain several copies of the chromosome and an organelle can contain more than one nucleoid. ( ) tend to have more nucleoids per organelle than mitochondria.

Animals / fungi & protists / plants

Rank the size of mitochondrial genomes in three classes of organisms (the first being the smallest)

Mitochondria

An organelle whose main function is oxidative phosphorylation, which is a process used to generate ATP (the energy source that drives cellular reactions) and consume oxygen. It contains DNA that consists of only 17,000 bp as well as rRNA and tRNA genes. 13 of its polypeptides function in oxidative phosphorylation. Most of these proteins are encoded by genes in the nucleus and are produced in the cytoplasm before being transported into the mitochondria.

Chloroplasts

An organelle whose main function is photosynthesis. The genetic material in this organelle is called cpDNA. It is typically 10 times larger than the mitochondrial genome of animal cells. It contains several genes, some of which are rRNA and tRNA genes, as well as many polypeptides that are required for photosynthesis.

Many of these proteins are encoded by genes in the nucleus and contain targeting signals that move them from the cytoplasm into this organelle.

Maternal Inheritance

The transmission of genetic material through chloroplasts or mitochondria from the mother to her offspring.

Mother / egg / wild-type / mutant / both

Fill in the blank…

An example of maternal inheritance….

Correns found that pigmentation of leaves in four o’clock plants could be green, white, or a mix of both. He found that this pigmentation depended solely on the ( ).

This occurred because the chloroplasts responsible for leaf color were transmitted only through the cytoplasm of the ( ), not the pollen grains. The type of chloroplast explained the left color found.

Green was the ( ) due to normal chloroplasts making the green pigment. White was the ( ) due to a mutation that prevented the synthesis of green pigment. The mixed phenotype resulted from cells with ( ) chloroplasts.

Heteroplasmy

When a cell contains both types of chloroplasts, resulting in a mixture of both phenotypes.

Ex) Variegated four o’clock plants. As they grow, the two types of chloroplasts are unevenly distributed to daughter cells. A cell may receive only white, resulting in all white sectors of the leaf. Or, it can receive all green or a mixture of the chloroplasts, resulting in all green sectors